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 The comparative anatomy of the teeth of 
 
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MEDICAL LIBRARY, 
 
 THE 
 
 COMPARATIVE ANATOMY 
 
 TEETH OF THE YERTEBRATA. 
 
 BY 
 
 JACOB L. WORTMAN, A.M., M.D., 
 
 ANATOMIST TO THE U. ^ARMY MEDICAL MUSEUM, WASHINGTON, u. C. 
 
 REPaiNTED FROM 
 THE AMERICAN SYSTEM OF DENTISTRY, 
 
 T 
 
 UU\VF-R9\TY 
 

 M*L^-. ^"1% 
 
THE COMPARATIVE ANATOMY OF THE 
 TEETH OF THE VERTEBRATA. 
 
 By JACOB L. WORTMAN, A. M., M. D. 
 
 A STUDY of the dental organs of the Vertebrata is one replete with 
 much interest when viewed from the standpoint of the naturalist. The 
 circvimstance that their modification is so intimately associated with tlie 
 food-habits of the animal, being principally concerned in the prehension 
 and comminution of the food, and that to these same habits we must 
 look for the most powerful influences and incentives to modification in 
 general, causes them to assume more than ordinary importance in the 
 estimation of the philosophic anatomist who earnestly addresses himself 
 to the problem of vertebrate evolution. 
 
 The fact, too, that the perfect condition in which they have been so ofteii 
 preserved in the fossiliferous strata of the earth's crust has frequently 
 furnished the only evidence which we possess of the existence of forms 
 long since extinct, causes them to be regarded as objects of still greater 
 interest. When we reflect that with nothing more to guide his judg- 
 ment than the dental series of an animal the expert palaeontologist can, 
 generally, not only indicate with great certainty the cliaracter of the 
 food upon which the animal subsisted, but its general characteristics 
 and relationships as well, even though the date of its existence be 
 removed to a remote period in geologic history, but little surprise 
 can be felt that so much thoughtful attention has been bestowed 
 upon this set of organs. 
 
 No series of anatomical structures lias proved of greater utility to the 
 systematist who has endeavored to indicate the exact relationship or 
 philogenetic history of mammalian forms than the teeth. Generally, 
 the student who attempts to master the subject is discouraged almost at 
 the very threshold of his undertaking by the apparently great diversity 
 of tooth-forms to be met with in the mammalian class ; but if looked 
 at from a developmental point of view, and if a little careful attention 
 is bestowed upon the plan of organization of the teeth of certain groups, 
 it is not difficult to discover that there are certain central or primitive 
 types from which it is easy to derive other related forms of dentition 
 by simple addition, subtraction, or modification of parts already pos- 
 sessed. 
 
 Careful attention to this subject for several years past, with the assist- 
 ance of the light which American palaeontology is now able to throw 
 upon the question, has convinced me more and more of the truth of 
 this assertion ; and I feel well assured that we are now in a position to 
 
 :151 
 
■In 
 
 35l> DENTAL ANATOMY. 
 
 lay down some broad principles in regard to dental e\'olution, at least 
 among certain groups of the Mammalia, where they have been subjected 
 to the greatest amount oi' modification. 
 
 Although there are many questions concerning the origin and details 
 of tooth-evolution of many al)errant forms which remain to be solved, 
 yet the discoveries whic'h have been made in palseontologv within the 
 last twenty-five years leave scarcely a living group of animals, the 
 development of ^\■h(lse teeth has progressed beyond the primitive stages, 
 from which -we have not gained some im])ortant information relative to 
 the phases through which they have passed to reach their present con- 
 dition. The possibility of reducing our knowledge of the dental struc- 
 tures of the RLuumalia to a l)road and comjirehensive basis was Ion, 
 since recognized by Prof. Coj)e, to wiiom probably more than any one 
 else we are indebted for a genuine philosophic insight into the forms 
 and structure of these teetli. Si'areely less important arc the contribu- 
 tions of John A. Hv'der and Dr. Harrison Allen, whose learned researches 
 into the probable causrs of tooth-modification \\aw marked notable 
 stagx's in the progress of the subjert and have o])cned new and inter- 
 esting fields for investigation. Nor should wt' omit a mention of the 
 researches of Flower, nor those of Tomes, A\'aldeyer, Frey, Hertwig, 
 Magitot, and Legros, into the histology and develojiment in later 
 times. 
 
 C'onmionly, teeth are defined as hard bodies attached to the parietes 
 of tile mouth or oral extremity of the alimentary canal, whose chief 
 function is the seizui'c and connuinution of the food. Morphologically 
 considered, however, tluy arc specialized dermal apj)endages situated in 
 the l)uccal cavity, and characterized l)y the ]>resence of certain calcified 
 tissue di;veIo|ied from the true derm or coriuni of the integument, 
 known as ilciiiliic. It will lie seen from this definition that the term 
 "tooth," strictly s])cai<ing, is limited to those structures of the oral 
 cavity -whicli alone ])ossess sucli tissue, although it is a recognized fact 
 that to other cjiithclial or cuticular structures, found in many inverte- 
 ))ratc and some few vertebrate forms, the term "tooth" has likewise 
 been applied. 
 
 ^\'hile tluy all sul)scrvc the same purjKise, and are therefore analo- 
 gous, their chief distinction consists in this — viz. in the latter, so far 
 as they have been investigated, tliese cirgans consist of a corneous or 
 horny substance, whicli is invariably derived from the more superficial 
 epideruiai layt'r, and is therefiire ecilcrnuic in origin. In the fin'mer 
 a, papilla arises from the corium, being sunk into a fi)ld oi' pit, and 
 eventually undergoes more or less calcification from its summit down- 
 ward by a dejiosition in its substance of lime salts, forming (hutiiic. 
 The dentine thus formed is a hard, elastic substance, consistini;: of 
 closely-set parallel tubuli, branching as they go, and wliose crown may 
 or may not be invested with an (>xceedingly hard and unyieldini^' sub- 
 stance derived from the deeper layers of the ej)idcrmis, known as entimd. 
 These are, then, cmlrroiiic in origin. 
 
 Those of eederonic source include the so-called teeth of Annul(.)sa% 
 Mollusca, Inseeta', et('. among the invertebrates, as well as the horny 
 teeth of OrnitlKirliijnchiis, palatal plates of the Siiriiia, and the horny 
 
TEETH OF THE VERTEBRATA. 
 
 353 
 
 teeth of the lampreys among vertebrates. If the term " tooth " 
 is applicable to these structures, then we must likewise include the 
 " baleen " of the Cetacea and the beaks of birds and rejitiles, wliifli by 
 common consent are far removed from true teeth. For all such I tliink 
 the term oral armature is preferable, from the tact that their produc- 
 tion not infrequently depends upon the modification of organs widely 
 different in origin. 
 
 On the other hand, those of enderonic sourcu are found only within 
 the limits of the Vertebrata, and range in form from the simple cone 
 usual among fishes to the higher complex grinding organs of certain 
 herbivorous mammals. The}' all agree in being developed from the 
 corium of the lining membrane of the mouth, which is continuous with, 
 and really a part of, the integument, invaginated at an early period. 
 There is a possible exception in the pharyngeal teeth of fishes, which 
 Ryder considers to be of hypoblastic origin or developed from the base- 
 ment-layer of the mucous membrane of the alimentary canal, and which 
 are practically the same as those of epiblastic origin, as far as their 
 relation to the surface is concerned. 
 
 When we speak of teeth being modified dermal appendages, it will 
 not be amiss to cite the evidence upon which such a genei'alization rests. 
 This is best afforded by a study of the relationship and development of 
 the dermal armature of certain elasmobranch fishes, of which the shark 
 is a good example and furnishes us with one of the earliest, and there- 
 fore one of the most primitive, conditions of the Vertebrata. 
 
 In these fishes the defensive power of the integument is augmented 
 by the production of numerous hard bodies in its substance, which have 
 been termed ." dermal denticles " by Gegenbaur. These structures, 
 which are likewise known as " placoid scales," are distributed over 
 the whole of the integument in shark-like fishes, and are ordinarily 
 
 Fig. 187. 
 
 Vertical Section through the Skin of an Embrj'onic Shark; c, corium; c, c, c, layers of corium; d, 
 uppermost layer; //.papilla; ^.epidermis; e, its layer of columnar cells; o, enamel layer (from 
 Gegenbaur, after Uertwig). 
 
 rhomboidal in form, with their apices directed obliquely backward. 
 They consist of a solid body, which is inserted by its base into the 
 
 Vol. I.— 23, 
 
354 DENTAL ANATOMY. 
 
 corium, with an exposed j^art, which is covered with a substance indis- 
 tinguishable from the enamel of the teeth. The structure of the body 
 is likewise coincident with true dentine, and becomes fused with a basal 
 plate of osseous material. Their development is as follows : First, a 
 papilla arises from the uppermost layer of the corium, being covered in 
 by the epidermis (see Fig. 187). From the deepest layer of the epi- 
 tlcrmis, or that which corresponds with the Malpighian layer, a special 
 epithelial covering is furnished, which eventually becomes, by a process 
 of histological difi'crentiation, the enamel of the exposed jDart. The 
 papilla, before the conversion of its substance into dentine, exhibits a 
 central cavity, from which fine branched canals radiate to the surface. 
 Eventually, calcification takes place, beginning at the summit, and the 
 salts of lime are deposited in the substance of the papilla, giving rise to 
 the dentine. Gegenbaur ( )bser\'es : ' " The placoid scale has therefore the 
 structure of dentine, is covered by enamel, and is continued at its Ijase 
 into a plate formed of osseous tissue ; as they agree with the teeth in 
 structure, they may be spoken of as dermal denticles." 
 
 Now, in the early embryonic stages the integument bearing these 
 dermal denticles is pushed into the oral cavity, Mhere they become 
 somewhat enlarged, and appear in the adult form as teeth. Tomes 
 says : ^ " Xo one can d(jubt, whether from the comparison of the adult 
 forms or from the study of the development of the parts, that the teeth 
 of the shark correspond to the teeth of other fish, and these again to 
 those of reptiles and mammals ; it may be clearly demonstrated that 
 the teeth of the shark are nothing more than highly-developed spines of 
 the skin, and therefore we infer that all teeth bear a similar relation to 
 the skin." Thus the generalization is reached that teeth are but spe- 
 cialized dermal appendages. 
 
 With this statement of the nature of teeth in general, we are now pre- 
 pared to begin a more special inquiry into the organization of a single 
 tooth. For this purpose I have selected the third lower pi'emolar of 
 the dog as an average and easily-procurable example of a generalized 
 type among the higher forms, which will serve to illustrate the compo- 
 sition and nomenclature of the several parts of AN'hich all teeth, ^^•itll few 
 exceptions, are made up. 
 
 For convenience of description, the several parts of most teeth can be 
 divided into crown, fang, and neck, although there are many in which 
 no true fangs are formed, owing to the persistent and continuous growth 
 of the tooth ; in all such no distinctions of this kind can be recognized. 
 In the particular tooth under consideration, ho«e\'er, \\q can distinguish 
 without difficulty an enamel-covered crown, which corresponds with the 
 exposed part of the tooth in the recent state ; two more or less cylindrical 
 fangs or roots, by which the tooth is implanted in the aveoli and attached 
 to the jaw bone; and a slight constriction at the point where the fangs 
 join the crown, known as the neck (see Fig. 18.S). The crown in form 
 resembles a laterally compressed cone, with an anterior and posterior 
 cutting edge. It is covered by a dense siiiny white substance of great 
 hardness, the enamel, which ceases at the point where the fangs com- 
 
 ' Elements of Comparative Anatomy. * ^ J\famial of Dental Analamu. 
 
TEETH OF THE VERTEBBATA. 355 
 
 mence. At the base of the crown the enamel is thrown into a conspic- 
 uous fold or ridge, which completely encircles the tooth at this point, and 
 is called the dngulum. Of the two cutting edges, the posterior is the 
 more extensive, and is interrupted in its descent from the summit of the 
 crown by a deep transverse notch, which constricts off „ , og 
 a prominent cusp known as the posterior basal tuber- 
 cle. A slight indication of a second cusp of this kind 
 is seen immediately behind it as an elevation of cingu- 
 lum. The anterior is the shorter, and descends from 
 the apex of the crown to the cingulum without inter- 
 ruption. It is placed nearer the inner than the outer 
 border of the tooth, and curves somewhat inward at 
 its lower extremity. 
 
 The fangs are two in number, occupying an antero- 
 posterior position, and give firm support to the crown. 
 They are covered by a softer substance, resembling bone- ^, . . , 
 
 ,' 1 "^ , , , /.I Third Lower Premolar 
 
 tissue, known as cementum or orusta petivsa ot human of h nog (Cants /a- 
 odontography. This material is continued over the ™""'"^'' '°'"8ed. 
 entire surface of the crown as an excessively thin stratum in the unworn 
 teeth of the Carnivora and several other orders, but can be demonstrated 
 only by the most delicate manipulation and the use of the microscope. It 
 assumes a more important relationship with the crown, as we shall pres- 
 ently see, in the herbivorous species of mammals. 
 
 Of the two fangs, the posterior is the larger, but the shorter, and takes 
 the greater share in the support of the crown, although the cleft which 
 separates them at their summits is placed directly beneath the summit 
 of the crown. It is broad at its base, and tapers somewhat abi'uptly to 
 an obtuse point. It is traversed by a vertical groove upon its anterior 
 moiety, which fits into a corresponding ridge on the side of its socket. 
 The anterior root is the more slender and the longer of the two. It 
 tapers more gradually, and is likewise traversed by a broad, shallow 
 groove upon its posterior aspect. At the point of each fang will be seen 
 a small aperture, the apical foramen, through which the nerves and 
 nutrient vessels pass to the pulp. 
 
 So far, we have spoken only of the external appearance of the tooth 
 and of those substances which make up its outer coverings ; but if both 
 the cementum and enamel were removed, it would still presetve its 
 original form, so gi'eat is the preponderance of the dentine as a constit- 
 uent element. This can best be seen in a longitudinal vertical section, 
 since at no part in an unworn tooth is the dentine exposed in these ani- 
 mals. Although the dentine is quite thick, and constitutes by far the 
 greatest part of the tooth, it nevertheless does not form a solid body ; 
 on the contrary, a considerable cavity is hollowed out in its centre, this 
 being largest in the part which makes up the body of the crown, and 
 extending down each fang. This cavity lodges the dentinal pulp, the 
 formative and nutrient organ of the tooth, and is in communication 
 with the exterior by means of the apical foramina of the fangs. 
 
 While this structure, in common examples of enamel-covered teeth, is 
 observable with the unassisted eye, a more minute study of the organiza- 
 tion of the various tissues must be conducted with the aid of the micro- 
 
356 DENTAL ANATOMY. 
 
 scope. This necessarily requires a considerable amount of experience 
 and skill in the manipulation and jjrejjaration of material, so that to 
 the unpractised oljserver a j^roper determination of the things -which one 
 may see is not always an easy matter. On this account I have chosen 
 to follow the conclusions of the recognized authorities, especially tlie 
 excellent treatise on dental anatomy by Charles S. Tomes, in this brief 
 statement of the histology, rather than trust the accuracy of my own 
 observations on the same. Since the histology of human teeth has been 
 more fully made out than perhaps the histology of those of any other 
 animal, it is here taken for illustration, although I am fully awai'e that 
 important deviations from the structure here described are to be met 
 with among the Vertebrata. 
 
 Dentine. — As we have already seen, the tooth consists of a dentine 
 body with a central cavity lodging the pulp, an enamel-capped crown, 
 and cementum-covered roots. The dentine is a hard, highly elastic, 
 translucent substance of a yellowish-'white tinge, ha^•ing a silky lustre 
 upon fracture. It is composed of an organic matrix highly impreg- 
 nated with calcareous salts ; through this matrix closely-set parallel 
 tubuli radiate from the pulp-cavity toward the periphery in a direction 
 at right angles to the surtace of the tooth. 
 
 Of perfectly dry dentine the following chemical analysis is given by 
 Von Bibra : 
 
 Organic matter (tooth-cartilage) . . 27.61 
 
 Fat . . . . 0.40 
 
 Calcium phosphate and fluoride 66.72 
 
 t'alcium carbonate 3.36 
 
 Magnesium pliosphate . 1.18 
 
 Other salts . . .83 
 
 The organic basis of the matrix, although closely related to that of 
 bone, is said not to be identical with it, and is hence called " dentine" or 
 " tooth-cartilage ;" it is perfectly structureless and transparent. After the 
 tooth has been decalcified by submitting it to the action nf dilute acid 
 for a few days, the matrix will still preser\'e the characteristic shape of 
 the tooth, and can readily be studied. 
 
 As already stated, tlie tubuli, which are like^^'ise known as denial 
 tubers, permeate the matrix in all directions, opening freely upon the 
 walls of the pulp-cavity, by whicli arrangement all 2>arts of the dentine 
 are brought into direct communication -with the central nutrient organ, 
 the pulp. They are mo.st nearly approximated and their diameters 
 greatest at their commencement on the walls of the pulp-cavitv, but, 
 pursuing a somefl'hat wavy course, gradually diminish in size, (jM'ing 
 to the numerous branches which they give ofP. These branches, although 
 not uniform in size, anastomose freely with thdse of the neighboring 
 tubuli, and frecpiently sho^v varicosities in their course. Tlie-s- termi- 
 nate either by gradually fading out, by anastomosing \\\i\\ trther 
 branches, by ending in loops, or by entering the enamel and cement- 
 um layers. 
 
 AMiile the dental tubes may be said to be channelled out in the sub- 
 stance of the dentine cartilage, the -walls of the tubuli are not formed 
 by this cartilage, but each tubuli is furnished with a structure knoM n as 
 
TEETH OF THE VERTEBBATA. 357 
 
 the dentinal sheath, which accompanies it througliout all its plexiform 
 radiations. The structure of these dentinal sheaths is not certainly 
 known, owing to the impossibility of isolating them -without decalcifica- 
 tion of the dentnie. Some histologists believe that they are calcified, 
 while others express doubt as to the correctness of this conclusion. One 
 very marked peculiarity which they possess is their great indestructibil- 
 ity. Dentine when submitted to the action of strong acid for a suf- 
 ficient length of time to completely destroy the intervening cartilage, 
 or when boiled in caustic alkali, will still exhibit these dentinal sheaths, 
 for it is indeed only in this way that their presence can be demonstrated 
 satisfactorily. One writer (Magitot) denies their existence altogether. 
 
 Enclosed within each dentinal sheath is a soft fibril, the dentinal 
 fibrils, which take their origin from the cells of the odontoblastic layer 
 of the pulp, presently to be noticed, and of which there are sufficient 
 reasons for believing them to be nothing more than processes or pro- 
 longations. There is, however, considerable discussion upon the exact 
 nature and relationship of these fibrils. Magitot maintains that they 
 are continuous with a layer of reticulate cells which lie beneath the 
 odontoblasts ; these freely communicate with processes of the odonto- 
 blasts, so that there is a very direct communication between the den- 
 tinal fibrils and the nerves of the pulp. He would therefore ascribe to 
 them a sensory function. Klein, on the other hand, holds that the 
 odontoblasts are concerned only in the formation of the dentine matrix, 
 and that the dentinal fibrils are long processes of deeper cells extended 
 between the odontoblasts. Whichever of the various views now held 
 may ultimately prevail, this much appears to be settled — viz. that the 
 dentine is extensively invaded, so to speak, by soft plasm ic material 
 derived from the pulp, by which it is not only nourished, but also ren- 
 dered highly sensitive. 
 
 In the outermost layer of the dentine, which underlies the cementum, 
 numerous globular spaces are found, in which many of the dentinal 
 tubes end ; these are filled with soft living plasma. These spaces, 
 if such indeed they may be properly termedj give to this layer a dis- 
 tinctly granular appearance, whence it was called by Tomes the " gran- 
 ular layer." Other structures, known as the interglobular spaces, pos- 
 sessing a ragged outline and short pointed processes, may frequently 
 be seen in dried sections of dentine. They are said by Tomes to be 
 most abundant at a little distance below the surface, and he believes 
 them to pertain rather to a pathological than to a normal condition. 
 
 The Tooth-pulp. — It appears best to describe in connection with the 
 dentine the pulp or formative organ, in consequence of the intimate 
 relation which exists between them. As has already been stated, it is 
 lodged in the pulp-cavity, and is the principal, if not the only, source 
 of blood- and nerve-supply to the dentine. In the young and growing 
 tooth, especially about the time calcification begins, it is largest and 
 assumes its greatest functional activity and importance, from the fact 
 that it is through its mediation that the dentine is formed ; in fact, in 
 the early stages of dental development, as we shall hereafter see, it is 
 coincident with the dentine organ itself, of which in the adult tooth it 
 is the inconsiderable remnant. As senile changes supervene it gradually 
 
358 DENTAL ANATOMY. 
 
 loses its formative energy, and may become entirely obliterated. Taken 
 at the adult stage of the tooth, it is seen to consist of indistinct finely 
 fibrous connective tissue containing numerous cells. The outermost 
 layer of the pulp is known as the vicmbrana eboris, and is made up of 
 a single layer of highly specialized cells of a dark granular appearance, 
 somewhat elongated, termed odontoblasts. These odontoblasts possess 
 large oval nuclei, and are provided with three sets of processes, as fol- 
 lows : the dentinal processes, M'hich are identical with the dentinal fibrils, 
 and, as we have already seen, enter the dental tubes ; the lateral pro- 
 cesses, by which they are connected with each other ; and, lastly, the 
 pit^j) processes, extending doM'n to a deeper layer of cells. This latter 
 layer of cells is somewhat intermediate in size between those more deeply 
 seated and the odontoblasts. Three or more arteries enter at the apical 
 foramen, and form a rich capillary plexus a slioi-t distance beneath the 
 membrana eboris. The nerves enter by several trunks along with the 
 arteries, and soon break up into a fine network in the substance of the 
 pulp. According to Boll, nerve-fibres penetrate the dentinal tubuli in 
 company with the dentinal fibrils, but this view is not fully accepted. 
 
 Cementmn. — The cementum inhuman and many other teeth of similar 
 structure may be said to be confined to the roots, investing them exter- 
 nally, unless the enamel cuticle or membrane of Nasmyth, mentioned 
 above, pertains to it, which C. S. Tomes and others believe to be the 
 case. It, like ordinary bone, consists of a gelatinous base combined 
 with calcareous salts, and is permeated by vascular canals. Its histo- 
 logical structure presents so many characters common to bone that it is 
 difficult to consider it anything more than a slight modification of that 
 tissue. Just as in bone, large irregular sjDaces (lacunce), filled with jiro- 
 toplasmic substance and presenting numerous minute radiating canals 
 (canalicuU), which anastomose with those of neighboring lacunae, are 
 found in ordinarily thick cementum ; certain differences are, however, 
 seen to exist. 
 
 The lacunae of cementum, for example, are more variable in size 
 and are noted for the great length of their canaliculi. The direction, 
 too, of the canaliculi is generally parallel with that of the dentinal 
 tubuli, radiating from two sides only, whereas in bone-tissue they 
 radiate in all directions. It has been already stated that the dentinal 
 tubuli sometimes enter the cementum layer. When this is the case 
 they become continuous with the canaliculi of the most deeply dis- 
 tributed lacunae. The outermost or granular layer of the dentine goes 
 so far toward establishing a complete transition in structure between the 
 cementum and the dentine that it is generally impossible to draw a 
 dividing-line and say where tlie one ends and the other begins. As to 
 limit of distribution of the cementum on the surface of the teeth in man, 
 monlvcys, carnivores, and insectivores, different views have been expressed, 
 owing to the various constructions that have been placed upon the nature 
 and relationship of the enamel caiticle or Nasmyth's membrane, already 
 mentioned. Waldeyer, Huxley, and KoUiker hold tliat it is no way 
 connected A\ith the cementum, but that it is a product deri\-ed from the 
 enamel, and is therefore epithelial in origin. C. S. Tomes, Magitot, 
 and AVedl, on the other hand, maintain that it is a part of the cementum 
 
TEETH OF THE VEBTEBBATA. 359 
 
 extended over the entire crown of the tooth, and becomes continuous 
 with its outermost layer in the vicinity of the neck. It is one of those 
 excessively thin membranes (not over -^-^-^ inch in thickness, accord- 
 ing to Kolliker) which are peculiarly indestructible and resist the ac- 
 tion of the strongest acids and alkalies. When stained with the nitrate 
 of silver, it shoAvs a peculiarly reticulated structure resembling epithe- 
 lium, which is believed by Tomes to be due to the pitted surface on its 
 interior, by which it is applied to the enamelrprisnis. Encapsuled 
 lacunte are likewise found in its substance, which would be difficult to 
 explain if it were not a part of the cementum layer. Tomes has like- 
 wise traced its connection with the outer layer of the cementum on sev- 
 eral occasions, and is therefore firmly of the opinion that it is a continua- 
 tion of this tissue. 
 
 Enamel. — The excessively hard, shiny substance investing the crown 
 of the tooth is the enamel. It is by far the hardest tissue to be met 
 with in the animal body, being at the same time the poorest in organic 
 constituents. Where it exists at all, it generally forms a cap of varying 
 thickness over the exposed part of the tooth, except in those instances 
 where there is an excessive development of cementum in this situation, 
 which causes it to occupy a position between the cementum and dentine, 
 as seen in the most exclusively herbivorous feeders, of which the horse, 
 cow, and elephant are good examples. Even here palseontological evi- 
 dence is quite conclusive in support of the proposition that their earlier 
 representatives possessed teeth with naked enamel-covered crowns. This 
 condition of nudity of the enamel is coincident with shorter cusps and 
 less elevated ridges of the crown, and, as we have good reasons to infer 
 from analogy, with more omnivorous habits of feeding. It can thus be 
 shown that this anomalous arrangement of the tissues is one acquired 
 comparatively late in the development of these forms for the exclusive 
 purpose of giving greater strength to the lengthened cusps, thereby 
 affording immunity from fracture during the act of mastication. 
 
 Von Bibra gives the following chemical analysis of the enamel of an 
 adult human tooth : 
 
 Calcium phosphate and fluoride 89.82 
 
 Calcium carbonate . 4.37 
 
 Magnesium phosphate . . . ' 1.34 
 
 Other salts 88 
 
 Cartilage . . 3.39 
 
 Fat 20 
 
 The proportion of the organic to the inorganic material is therefore 3.59 
 to 96.41, while in dentine it is 28.01 to 71.99. Its structure consists 
 of minute hexagonal prisms, known as enamel-fibres or enamel-prisms, 
 whose long axes, broadly speaking, have a direction at right angles to the 
 surface of the tooth. It is a comparatively rare occurrence to find the 
 fibres pursuing a perfectly .straight course from the dentine to the sur- 
 face, but such is found to be the case in the enamel of the manatee or 
 sea-cow and several other forms. Usually, they are tortuous, and fre- 
 quently decussate, as in the human subject, which rendei-s it difficult to 
 trace the course of an individual fibre. A variety of patterns is pre- 
 
360 DENTAL ANATOMY. 
 
 sentetl by the arrangement of these prisms in the enamel of different 
 animals, esj)ecially of the " gnawing quadrupeds," or rodents. 
 
 The prisms, when decalcified and isolated, exhibit slight ^•aricosities 
 or enlargements, giN'ing them a distinct transversely striated appearance, 
 not unlike that of voluntary muscular fibres. Tliey are olherM'ise 
 structureless. It is maintained by Bodecker that the prisms are not 
 absolutely in contact, but that minute spaces exist between them which 
 are filled with active protoplasmic material, "which becomes continuous 
 with that of the dentinal tubuli, thereby furnishing a means of nutrition. 
 Some investigators admit this interstitial substance, but attribute to it 
 no greater function tlian that of simple cementing material, while others, 
 again, claim that the j^risms are in absolute contact, and that no inter- 
 vening substance is demonstrable. Owing to the disparity in extent 
 between the outer and inner surface of the enamel, as well as the fact 
 that the individual prisms do not decrease in size nor brancli in their 
 coiu'se outward to the surface, considerable spaces would be left if it were 
 not that they are occupied by numerous prisms which do not penetrate 
 to the dentine. The prisms end in sharp-pointed extremities which are 
 received into corresponding pits in the enamel cuticle or inembrane of 
 Nasniyth. 
 
 Devei.opjient. — Xext in. order Avill Ije briefly noticed the develop- 
 ment, so as to complete in this connection an entire statement of the 
 anatomy of a single tooth. It may be said that altliough teeth (if dif- 
 ferent types differ to a \\onderful degree in their forms, which would 
 seem to indicate diffisrences quite as great in other respects, yet, in 
 fact, tlie plan of their development is substantially the same -wlierever 
 found. iS<.i far is this true that the description of the embryology of 
 one tooth will, with little modification, answer fairly well for all teeth. 
 The more important of tlicse modifications in the details of development 
 will be discu.ssed in connection with the teeth of the various subdivis- 
 ions of the Vertebrata. 
 
 AA'e have already stated that the teeth are derived from the lining 
 membrane of the oral cavity, \\liicli blends with the integument at the 
 lips. The principal differences lietwecn the integument Mhich covers 
 the surface of the body and the mucous membrant^ ^^-hich lines the ali- 
 mentary canal are those of function and origin, the sti-ucture being 
 essentially the same. In the one the individual cells of the epidermal 
 layer become devitalized and scale oft', while in the other they are 
 actively engaged in the secretion of mucous, gastric, intestinal, and 
 other juices during alimentation. The devitalization and consequent 
 "shedding of the skin" is greater in some forms than in others. In 
 the frogs and salamanders, for example, the skin is kept constantly 
 moist by an abundant mucoid secretion, and the epithehum of the integ- 
 ument may be said to be more "alive" in these animals than in birds, 
 reptiles, or mammals. The difference in origin consists in the import- 
 ant fact that the integument is formed from the epiblastic or outermost 
 layer of primitive embryonic growth, while the mucous membrane of 
 the alimentary canal is derived from the hypoblastic or innermost laver 
 of the same. In the early stages of the development of the embryo the 
 skin is more or less invaginated into the mouth-cavitv, and partakes 
 
TEETH OF THE VERTEBBATA. 361 
 
 somewhat of the nature of mucous membrane proper. The real point 
 of blending is, in the embryo at least, not at the lips, but lies inside the 
 borders of the jaws. If, therefore, we limit the term " mucous mem- 
 brane" in this situation to that tissue Avliich is of hypoblastic origin, 
 then the teeth of the jaws cannot be said to be developed from the 
 mucous membrane of the mouth, as is commonly stated, but from the 
 invaginated integument. 
 
 In many fishes teeth are found far back in the pharynx, and are 
 attached to the gill-arches and pharyngeal bones. I am informed by 
 Mr. J. A. Ryder, whose extensive knowledge of the embryology of 
 fishes renders his statements highly authoritative, that these teeth lie 
 beyond the limits of the invaginated integument, and are truly of hypo- 
 blastic derivation. If this be true, the generalization that all teeth are 
 modified dermal spines is certainly incorrect. It affords us, however, 
 an example in which identical structures have been produced from tissue 
 of vastly different origin in a similar manner, and in all probability 
 attributable to the same causes — viz. repeated stimulation of a particu- 
 lar point, which eventually gave rise to a calcified papilla. 
 
 The point at which a tooth is about to be developed is marked by a 
 proliferation of the cellular elements of the tissue in which it will ulti- 
 mately appear. These eventually arrange themselves into three organs, 
 which have been denominated the dentine organ, the enamel organ, and 
 the dental saccidus. This latter organ becomes so modified in some ani- 
 mals, in which coronal cement is extensively developed, as to merit the 
 distinction of oementum organ. Taken collectively, they represent the 
 tooth-germ. C. S. Tomes very justly remarks that "the tooth is not 
 secreted or excreted by the tooth-germ, but an actual metamorphosis of 
 the latter takes place." The three principal tissues, dentine, enamel, 
 and cementum, thus produced, are formed from their respective organs, 
 and consequently separate parts of the tooth-germ. Although many 
 adult teeth do not possess enamel upon their crowns (e. g. edentates or 
 sloths, armadillos, etc.), yet the presence of an enamel organ in the early 
 stages of growth is believed to be a universal feature of the development 
 of all teeth, and is one of the strongest arguments for their community 
 of origin, however much they may have been subsequently modified. 
 
 The Enamel and Dentine Organs. — In the earliest stages of the 
 development of a mammalian tooth, which is here taken for descrip- 
 tion, a slight longitudinal depression in the epithelium covering the bor- 
 ders of the jaws is noticeable ; this is somewhat augmented in dejrth by 
 the addition of a ridge upon either side of it. At the bottom of this 
 groove the deepest or Malpighian layer of the epithelium grows down 
 into the corium as a continuous fold or lamina, being directed down- 
 ward and a little inward. In cross-section this fold resembles a tubu- 
 lar gland and extends throughout the entire length of the jaw. In the 
 positions where teeth are to be formed the lower extremity of this 
 lamina is considerably enlarged by the rapid multiplication of its con- 
 stituent cells. The continuity of the fold is now broken up, and the 
 'structure which is destined to become the enamel organ appears as a pro- 
 cess of epithelium comparable in shape to a Florence flask (Fig. 189). 
 The outermost layer of the organ at this stage is made up of cells of 
 
362 
 
 DENTAL ANATOMY. 
 
 Fig. 189. 
 
 the columnar variety which still retain their connection with the Mal- 
 pighian layer above, from which they were orignally derived, while the 
 
 interior of the enlarged ex- 
 tremity is composed of polyg- 
 onal cells. 
 
 As development proceeds, the 
 edges of the enlarged extremity 
 grow more rapidly downward 
 than the centre, which causes it 
 to assume a bell-shaped form, 
 with the concavity directed 
 downward. Synchronous with 
 this growth, a papilla arises 
 from the corium beneath and 
 is closely invested by the enamel 
 organ. The appearance of this 
 papilla marks the earliest stage 
 in the development of the den- 
 tine organ, but it will be well 
 to examine more closely at this 
 stage the structure of the enamel 
 organ. While it retained the 
 shape of the Florence flask its 
 periphery consisted of colum- 
 nar epithelium, the interior be- 
 ing made up of polygonal cells. 
 Coincidentally with its assump- 
 tion of the bell shape those cells 
 of the peripheral layer which are 
 brought into juxtaposition with 
 the dentine bulb or organ un- 
 dergo great elongation and en- 
 largement, forming very regular 
 six-sided prismatic bodies, and 
 are known as the enamel-cells. 
 The polygonal cells of the interior are transformed into a stellate retic- 
 ulum composed of cells with remarkably elongated processes ; these pass 
 through a series of unaltered cells known as the stratum intermedium into 
 the enamel-cells. Lastly, wc have the outer layer, which is little changed, 
 and still remains connected with the Malpighian layer by a slender cord 
 of epithelium. This layer is called the external epithelium of the 
 enamel organ. 
 
 Before the dentine papilla makes its appearance " a dark halo," more 
 vascular than the surrounding parts and corresponding to the epithelial 
 lamina or fold which gives rise to the enamel organ, is to be seen in the 
 submucous tissue or corium. Immediately beneath the enlarged ex- 
 tremity of the enamel organ the dentine papilla is developed at about the 
 time this stage is reached by the enamel organ. In its peripheral laver 
 highly specialized cells with several sets of processes, odontoblasts — 
 already described in connection with the tooth-pulp — make their appear- 
 
 Three Stages in the Development of a ^^laramalian 
 Tooth-germ; a, oral epithelium heaped up over germ; 
 h, younger epithelial cells ; c, deep layer of cells or 
 rete Malpighii ; rf, inflection of epithelium for enam- 
 el germ: e, stellate reticulum;./', dentine gerra ; g, 
 inner portion of future tooth-sac; ?i, outer portion 
 of future tooth-sac ; i, vessels cut across ; k, bone of 
 jaw (from Tomes, after Frey). 
 
TEETH OF THE VERTEBBATA. 363 
 
 ance, while in the remainder of the bulb numerous other cells, identical 
 with those of the tooth-pulp, are developed. It also becomes highly 
 vascular. Very soon the odontoblasts nearest the surface undergo 
 metamorphosis into a gelatinous matrix, and their nuclei disaj)pear; 
 they are next calcified from the summit downward, and we soon recognize 
 a thin dentine cap over the entire bulb, Avhich gradually increases as 
 development proceeds. The central portions of the odontoblasts remain 
 uncalcified and form the dentinal fibrils, while the lateral processes occa- 
 sion the numerous anastomoses of the dentinal tubuli and fibrils seen in 
 the adult tooth. The dentine mass is gradually thickened by successi\e 
 increments from within by a repetition of the process above described, 
 so that it will thus be readily seen that the configuration of the dentine 
 body, and consequently the entire tooth, is established as soon as calcifi- 
 cation has fairly set in. 
 
 Returning to the enamel organ, we can now briefly follow its devel- 
 opment to completion. We have already seen that it consists of an 
 outer layer of columnar epithelium covering the convex portion, and is 
 connected by a slender cord with the Malpighian layer above. It con- 
 sists also in part of an internal stellate reticulum which passes by means 
 of a layer of rounded cells (stratum intermedium) into the enlarged, 
 greatly-elongated prismatic cells lining the concave lower surface, which 
 invests the dentine organ like a cap. Before the enamel is completed 
 the external epithelium, the stellate reticulum, and stratum interme- 
 dium disappear altogether, but before this atrophy takes place the neck 
 or epithelial cord of the enamel organ gives rise to the tooth-germ of the 
 permanent tooth as a diverticulum which is developed in the same way 
 as the germ of the first or deciduous tooth just described. 
 
 The essential part of the enamel organ, or rather that which ulti- 
 mately results in the formation of enamel, consists of enamel-cells. 
 These, as we have said, become greatly elongated and assume the form 
 of regular hexagonal prisms, which agree in shape with the calcified 
 enamel-prisms of the complete tooth. Just as in the odontoblasts of the 
 dentine, they are transformed into a gelatinous matrix, the nucleus dis- 
 appears, and calcification begins from above, the only difference being 
 that the enamel-prisms calcify completely, and are therefore not tubular, 
 while in the corresponding structures of the dentine dentinal tubuli are 
 left. Different views have been advanced in regard to the exact desti- 
 nation as well as the function of the several parts of. the enamel organ 
 spoken of above as disappearing by atrophy. As to the fate of the 
 external epithelium, Waldeyer holds that after the disappearance of 
 the stellate pulp it becomes applied to the outer surface of the enamel 
 as the membrane of Nasmyth, which would certainly seem to be its 
 most natural fate ; but Kolliker, Magitot, and Legros claim, on the 
 other hand, that it disappears altogether. Most authors believe that 
 the enamel organ is devoid of vascularity, but Beal asserts that there is 
 a vascular network in the stratum intermedium. If it be non-vascular, 
 then it is more than jjrobable that the pulp represents stored-up pabulum 
 from which the requisite formative energy is derived. If vascular, it 
 then probably subserves a mechanical purpose only, as some authorities 
 believe. 
 
364 DENTAL ANATOMY. 
 
 The Dental Sacculus and Cement Organ. — So far, no mention has 
 been made of the development of the dental sacculus. At an early 
 period in the growth of the dentine j^apilla a process of the submucous 
 tissue arises ti'(jm its base and seems to grow ujjward on the outside of 
 both dentine and enamel organs, finally coalescing on top, so as to enclose 
 the growing tooth-germ in a shut sac, the dental sacculus. Whether 
 there is an actual growth of processes from the base of the dentine bul-b, 
 or ^^'hether the adjacent c(.)nnective tissue is transformed into it, appears 
 not to have been ver)- accurately determined ; at all events, the con- 
 nective tissue immediately in contact with the germ soon becomes 
 distinguishable from that external to it liy becoming richer in cells, 
 vessels, and fibrillar elements. A\'hen the sacculus is fully formed, it 
 is made up of an outer and an inner wall, both richly vascular. The 
 outer wall becomes the dental periosteum, while in the inner wall, 
 especially in the vicinity of the roots, osteoblasts appear and are calci- 
 fied into cementum, as in the formation of ordinary bone-tissue. Its 
 close application to the surface of the enamel, and partial or imperfect 
 calcification in most teeth, give rise to the membrane of Nasmyth. In 
 those animals, however, in which coronal cement is formed, such as the 
 Herbivora, there is developed in connection with the inner wall, between 
 it and the enamel, a fibro-cartilaginous structure containing character- 
 istic cartilage-cells. These undergo calcification in a manner not dif- 
 ferent from that seen in the formation of cartilage bone, and produce the 
 cementum in the teeth of these animals. It is then known as the 
 cementum organ. 
 
 V\& have now made clear, we trust, as complete a statement of the 
 anatomy of a single tooth as is consistent with brevity, but which will 
 serve as a basis for the comprehension of the more special part of our 
 subject — viz. the morphology of the teeth in the various subdi%'isions of 
 the Vertebrata. 
 
 The Accessory Organs — the Teeth, their Structure, Devel' 
 OPMENT, Replacement, and Attachment, in Pishes. 
 
 It will be impossible to gain anything like a concise understanding 
 of the dental organs of this extensive assemblage of vertebrate forms 
 until we have first briefly outlined their classification. In this I have 
 followed Pi'of Gill, believing that his interpretations more nearlv coin- 
 cide with a natural arrangement. 
 
 It is a common practice of naturalists to consider the A^ertebrata as 
 divisible into five classes, as follows : Pisces, or fishes ; Batrachla, or 
 frogs, salamanders, etc. ; Reptilia, or snakes, turtles, lizards, etc. ; Aves, 
 or birds ; and Mamriudia, or mammals ; but according to Prof Gill 
 there are differences quite as great, if not greater, between certain mem- 
 bers of the old class Pisces as there are, for example, bet-\veen some fishes 
 and frogs. For this reas<_in he divides the permanently gill-bearing ver- 
 tebrates, or those which aerate the blood throughout the entire life of the 
 individual by means of specially adapted organs known as " gills," into 
 four classes, which he defines as follows : 
 
TEETH OF THE VEBTEBRATA. 365 
 
 I. Skull undeveloped, with the notochord persistent and extending to the anterior 
 end of the head. Bi-ain not distinctly differentiated. Heart none. 
 
 Leptocardii. 
 II. Skull more or less developed, with the notochord not continued forward beyond 
 the pituitary body. Brain difterentiated and distinctly developed. Heart 
 developed and divided at least into auricle and ventricle. 
 
 A. Skull imperfectly developed, with no lower jaw. Paired fins undeveloped, 
 
 with no shoulder-girdle nor pelvic elements. Gills purse-shaped. 
 
 Maesipobeanchii. 
 
 B. SkuU well developed, witli a lower jaw. Paired fins develoj)ed (sometimes 
 
 absent through atrophy), and with shoulder-girdle (lyriforra or furcula- 
 shaped, curved forward, and with its respective sides connected below), and 
 with pelvic elements. Gills not purse-shaped Lyrifeea. 
 
 a. Skull without membrane bones ("a rudimental opercular bone" in Cliimmra); 
 
 gills not free, the branchial openings slit-like, usually several in number; 
 exoskeleton placoid, sometimes obsolete ; eggs few and large. 
 
 Elasmobkanchii. 
 
 b. Skull with membrane bones ; gills free ; branchial openings a single slit on each 
 
 side, sometimes confluent ; exoskeleton various, not placoid ; eggs compara- 
 tively small and numerous . ... Pjsces. 
 
 The first of these classes, Leptocardii, includes a few small fish-like 
 animals, such as the well-known amphioxus or lancelet occurring on our 
 coast, in which no skull exists. They are in many ways most remark- 
 able forms, being the most primitive of all vertebrates, but as they are 
 devoid of teeth, this class can be dismissed without further consideration. 
 The next, Marsipobranchii, embraces the lampreys, whose "horny teeth" 
 have already been alluded to. The relationship as well as examples of 
 each order of the remaining two classes is expressed in the subjoined 
 table (p. 366), which is compiled from Dr. Gill's papers on the 
 classification of fishes. 
 
 The Accessory Organs. — A consideration of thefse organs necessarily 
 involves not only a study of the bones and cartilages taking share in 
 the boundary of the oral cavity, but of all bones and cartilages in connec- 
 tion with which teeth are developed. It would likewise properly include 
 a mention of the muscles which move these parts, together with the vas- 
 cular and nervous supply ; but owing to their great range of variation, 
 as well as the limited space at my disposal, these latter will not be con- 
 sidered. This, in my judgment, is best accomplished by describing the 
 normal arrangement in some typical fish and comparing all others with 
 it. For this purpose a gadoid fish, or one of the cod tribe, is most suit- 
 able, since it exhibits the structure which obtains in a large majority of 
 ichthyic forms. 
 
 If a well-cleaned skull be examined, it will be seen to consist, in the 
 first place, of a cranium or brain-box, or that part which remains intact 
 after the skull has been boiled or macerated a sufficient length of time 
 to cause the soft parts to disappear and the arches and appendages to 
 become disarticulated. This contains the brain, and becomes continuous 
 at its lower back part with the vertebrae or axial pieces of the body skele- 
 ton into which the spinal cord passes. Suspended from either side of its 
 posterior portion there is a chain of bones which extends down beneath 
 the throat and bears the pectoral fins ; this is known as the shoulder-gir- 
 dle or scapular arch (see Fig. 190). 
 
 A short distance in front of this, or at a point about midway between 
 the root of the scapular arch and the eye-socket, another arch springs 
 
366 
 
 DENTAL ANATOMY. 
 
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TEETH OF THE VERTEBRATA. 
 
 367 
 
 from the side-wall of the cranium and passes downward and forward to 
 the proximal portion of the lower jaw, which is attached to it ; this is 
 known as the hyo-mandibular arch (see Fig. 204). Attached to the pos- 
 
368 DENTAL ANATOMY. 
 
 terior portion of this arch are several broad, flat, scale-like bones which 
 cover the gills and are called opercular bones. The upper posterior one 
 is the opemdum. The one in front of this, presenting a curved outline 
 anteriorly and a posterior serrate border, is the preopemdum , while the 
 two beneath are the iideropercxduvi and yubopciTidinn respectively. The 
 arch itself is composed, iirst, of the hi/u-viimdibidar bone (Fig. 190, 
 hi/), which by its proximal extremity is attached to the side-wall of the 
 cranium, being lodged in a distinct oblong scjcket ; secondly, of the 
 qiiijilrafe (qu, Fig. 190), which articulates with it by suture at its lower 
 extremity ; thirdly, the sympledic, a small .^jilint occ iij)ying a groove 
 in the inner side of the quadrate ; and, lastly, the lower jaw, which is 
 movably articulated with the quadrate and whicli normally supports 
 teeth. Each half is made up of the dentary or tooth-bearing piece, 
 meeting its fellow of the opposite side in the median line or symphysis, 
 and an articular piece which connects the dentary with the quadrate. 
 To this may be added the coronoid, a small bone superimposed above 
 the junction of the articular and dentary, and an amjular which lies 
 just beneath the articulation of the quadrate and articular. 
 
 From the region of the quadrate another chain of bones extends 
 upward, forward, and inward to the anterior part of the roof of the 
 mouth, where it is attached l^v ligament to the side of the vomer, or 
 that bone which forms the prominent rostrum of the cranium after the 
 removal of the arches. This chain is known as the jxdato-quadrate 
 arch, and the bones entering into its composition are the ento-, meso, 
 ecto-pterygoids and the palatine. The ento-jrtervgoid is applied to the 
 hyo-mandibular and quadrate upon their anterior margins ; the meso- 
 pterygoid starts out from the quadrate and ento-pterygdid, and extends 
 toward the vomer, where it meets the palatine, which completes the 
 arch. The ecto-pterygoid lies above the junction of the meso-pterygoid 
 and the palatine (Fig. 190). 
 
 Immediately in front of the vomer, and attached to it and to the pala- 
 tines, are two considerable bones which project do^ynward and backward, 
 bounding the upper posterior portion of the canthus of the mouth — the 
 ifiiperior mri.rillaricfi. In front of these, again, are the pre- or ivtermax- 
 i/lariex, limiting the anterior boundary of the oral cavitv above. 
 ^Vnother bone, which in some forms (ex. catfishes) reaches the roof of 
 the mouth, needs to be noticed in this connection. The suborbital 
 ring, or tliose bones wliicli encircle the orbit below, articulates by its 
 most anterior piece (lachrymal) with a bone suturally united to the 
 cranium and taking part in the boundary of the orbit in front and 
 above. This is the prcfrontrd, and, as already remarked in tlae cat- 
 fishes, owing to the width of the mouth takes part in the formation of 
 its bony roof, and in some species bears teeth. This bone is frequently 
 mistaken for the vomer, but, as I have recently ascertained, is certainlv 
 the prefrontal, which must likewise be added to the category of tootli- 
 supporting bones in fishes. 
 
 The several arches and bones so far enumerated, witli the exception 
 of the scapular arch — which never, to my knowledge, is dentigerous — are 
 in direct relation with the mouth, and arc exclusively concerned in pre- 
 hensile and crushing functions; but those which are to follow, especially 
 
TEETH OP THE VERTEBBATA. 369 
 
 the branchial arches, were primarily used in connection with respira- 
 tion, so that any relations with the teeth which they may have subse- 
 quently acquired must be looked upon as a secondary modification. 
 This peculiarity, moreover, is of such wide application in the class 
 Pisces that a description of these parts cannot well be omitted in a con- 
 sideration of the accessory organs. 
 
 The hyo-branchial skeleton lies beneath the base of the cranium, and 
 is pretty well concealed in a side view by the opercular, hyo-mandibular, 
 and quadrate bones. It is connected with the rest of the skull at two 
 points — viz. by the articulation of the stylo-hyal bone with the hyo- 
 mandibular, and the other by means of loose connective tissue which 
 binds the upper portion of the branchial arches to the base of the cra- 
 nium. Its general structure will be best understood by describing it as 
 composed of a series of transverse bony arches placed one in front of the 
 other, rising up from the floor of the mouth and meeting in the median 
 line above. 
 
 The most anterior of these is the hyoid arch, which is formed by two 
 median basilar pieces upon either side, the basi-hyak. Passing from 
 within outward, we have first the cerato-hyals, to which are appended 
 the branchiostegal rays. The next piece in the arch is the epi-hyal, fol- 
 lowing which is the stylo-hyal. This latter bone is a slender rod, and 
 serves to complete the connection of the hyoid arch with the hyo-man- 
 dibular bone. From the interval between the two most anterior basi- 
 hyals there projects a small bone forward which supports the tongue, and 
 is hence called the ento- or hyo-glossal. Projecting backward from the 
 inferior surface of these same basi-hyals is another piece, the uro-hyal. 
 
 Behind the hyoid, and similarly composed, are the five branchial 
 arches, of which the last two are somewhat modified. The three ante- 
 rior ones are made" up of median basilar bones, the baf<i-branchihyals. 
 With these are articulated the hypo-branohials upon the outside, after 
 which follow the cerato-branchial and epi-branchial pieces. In the 
 fourth branchial arch, counting from before backward, the hypo- 
 branchials are absent, and the uppermost segments are considerably 
 dilated and support teeth ; they are then known as the superior pharyn- 
 geal bones. The fifth arch is quite rudimentary, containing only the 
 cerato-branchial elements, which are generally much enlarged and bear 
 teeth ; these are the inferior pharyngeal bones. 
 
 The arrangement here described is found without substantial modi- 
 fication except as regards relative size and the degree of ossification of 
 the several parts in nearly all the sub-class Teleostei. In the Elasmo- 
 branchii, however, the skull remains largely cartilaginous, and the hyo- 
 mandibular arch is always more or less imperfectly represented. The 
 maxillae and premaxillse are likewise absent. In the chimeroid division 
 (Holocephali) neither the hyo-mandibular nor quadrate elements can be 
 made out, the mandible being attached directly to a broad triangular 
 cartilaginous lamella which stretches out from the sides of the base of 
 the skull, and whose anterior part bears the teeth of the upper jaw. It 
 will thus be readily understood that this cartilaginous plate, continuous 
 with the chondro-cranium, represents both the undifferentiated upper 
 portion of the hyo-mandibular and all of the palato-quadrate arches. 
 
 Vol. I.— 24 
 
370 DENTAL ANATOMY. 
 
 In the Plagiostomi (sharks and rays), on the other hand, a separate car- 
 tilaginous element representing the hyo-mandibular bone is always pres- 
 ent, and affords an articular surface to the lower jaw or mandible, which, 
 moreover, in all the elasmobranchiates consists of a single cartilaginous 
 bar, the primiti\e Meckelian cartilage. The palato-quadrate arch is 
 likewise present and forms the dentigerous border of the upper jaw (see 
 Fig. 204). Since the hyo-branchial skeleton in these forms is not con- 
 cerned in the support of teeth, it can be dismissed without further 
 mention. 
 
 The principal deviations in the structure and relationship of the den- 
 tigerous apparatus from the typical teleostean one to be met with in the 
 sub-class llnnoidei are furnished by the Dipnoi and Chondroganoidei. 
 The former of these orders includes the three living genera Ceratodm, 
 Protopterus, and Lepidosiren of Australia, Africa, and South America 
 respectively. They are most remarkable and interesting representatives 
 of types in some respects low down in the scale of ichthyic organization, 
 while in others high, in that they furnish many transitional characters 
 between true fishes and the Batraehia (frogs and salamanders). It is 
 highly probable that from some as yet undiscovered relative of this 
 group the Batraehia have been derived by descent. 
 
 In this order the skull is devoid of both maxillse and premaxillae, 
 and, as in the chimeroid elasmobranchiates, the hyo-mandibular arch is 
 not completely differentiated, the lower ja\A- being articulated directly to 
 the cranium. There is, however, a well-defined palato-quadrate arch 
 supporting teeth. The hyo-branchial skeleton, although resembling the 
 teleostean type of structure considerably, is edentulous. In the Chon- 
 droganoidei (sturgeons) the skull as well as the arches remain largely 
 cartilaginous. The suspensorium (proximal part of the hyo-mandibular 
 arch) presents two elements, usually homologized with the hyo-mandib- 
 idar and quadrate jjieces of the teleostean skull ; the latter of these 
 pieces affords attachment to the mandible. There is also a palato-quad- 
 rate arch. Only one species of this group, the shovel-nose sturgeon, 
 possesses teeth, and these, according to Owen, apjjear only in the young. 
 
 The remainder of the G<moidei agree with the Teleostei in the structure 
 and arrangement of the accessory organs. The latter sub-class, how- 
 ever, exhibits numerous minor variations, which are confined principally 
 to modifications of the hyo-branchial skeleton, such as the loss or atrophy 
 of certain of its component elements ; these are so numerous and varied 
 in their nature that it would be impossible, ajid quite foreign to the 
 object of the present article, to enumerate them. 
 
 Teeth of the Elas)nobrnrichU. — As already observed, this class is divis- 
 ible, not only by the differences which obtain in the arrangement of the 
 several arches, l)ut by the disposition, structure, and manner of replace- 
 ment of the teeth, into two primary groups, of which the sharks 
 and rays constitute one, and the Chimserfe tjie other. Of these, the 
 former is the more primitive, and in all probability gave origin to the 
 typical fishes, while the latter reseml)les more closely the dipnoans, and 
 may indeed prove to have been their ancestors. 
 
 The teeth of the sharks are always numerous, and are pre-eminentlv 
 adapted to the predaceous habits of their possessor. They are borne 
 
TEETH OF THE VERTEBRATA. 371 
 
 upon the cartilaginous mandibuli and palato-quadrate arches, being 
 attached not to the cartilages themselves, but to a thick, dense fibrous 
 membrane which forms an external investment. They are arranged in 
 concentric rows on the summit and inner surface of the jaws, being 
 developed from the bottom of a longitudinal fold of the lining membrane 
 in this situation, known as the thecal fold. The teeth of the upper- 
 most row, or those occupying the margins of the jaws, stand upright 
 and do service as the functional ones until discarded ; those of the next 
 row, as well as all the succeeding ones, usually occupy a recumbent 
 position, with their apices directed downward or upward according as 
 they belong to the upper or lower series ; but it not unfrequently hap- 
 pens in some species that the second, and even the third, rows may 
 exhibit different degrees of erection. As a general rule, but a single 
 row of teeth are in use at one time. The individual teeth composing 
 the longitudinal rows may be disposed with reference to those of the 
 succeeding ones so as to be parallel vertically, as is well exemplified 
 in the genus Lamna, or they may be placed in such a manner as to 
 alternate with each other, a condition seen in the blue shark (^Oar- 
 charias). As would naturally be surmised from this arrangement, the 
 way in which succession takes place is for the row beneath to rise up 
 and take the place of those in use. This is accomplished by the fibrous 
 gum in which their bases are imbedded sliding bodily over the curved 
 surface of the jaws from within outward, continuously bringing fresh 
 rows into position, as was long since demonstrated by Prof. Owen. 
 
 It thus happens, on account of this peculiar and, in my judgment, 
 remarkably primitive manner of succession, that large numbers of teeth 
 little worn are cast off during the life of each individual, and that 
 replacement goes on far in excess of the actual requirements of the ani- 
 mal, and quite independently of their temporary use as organs of pre- 
 hension and mastication — a fact which in itself demonstrates their der- 
 mal relationship. The only assignable cause for this extravagant devel- 
 opment of teeth, it appears to me, is due to inequalities in the rapidity 
 of growth in different parts of the body, which causes the integument 
 invaginated during embryonic development to be restored or evaginated 
 during adult growth. If this h}q3othesis be correct, then the whole 
 question of the force concerned in the succession of the teeth is reduced 
 to the simple explanation of inequalities of gro^vth primarily, however 
 much it may have been subsequently complicated and obscured in the 
 higher forms. Looked at from this standpoint, it is not such an 
 inscrutable mystery as C. S. Tomes and others would have us believe. 
 
 Considerable variety of form exists in the teeth of the different species ; 
 they may be heterodovi (that is, different in various parts of the jaws) ; 
 isodont (alike throughout) ; or hemihoviodont (in which the individual 
 teeth of the lower jaw are alike, but different from those of the upper 
 jaw, and reciprocally). In all, the teeth nearest the back part of the 
 mouth are smaller than those in front. The simplest form to be met 
 with is the unmodified cone with a sharp point and a broad base. Such 
 is found in the large Rhinodon and some " dog-fishes ;" to this may be 
 added basal denticles, as in the genus Lamna; or it may have a com- 
 pressed triangular outline with serrate edges, as in the upper teeth of 
 
372 
 
 DENTAL ANATOMY. 
 
 the blue shark (Carcharias). These lateral serratures may become so 
 strongly developed as to give to the tooth a distinct comb-like appeai'- 
 ance — e.g. lower teeth oi' Notidanus (Fig. 191). 
 
 Fiu. 191. 
 
 Teeth of Notidanus (after Gunther). 
 
 The most remarkable modification in the dental organs of sharks is 
 exemplified by the Port Jackson t^hark ( Ccstracion), in which the poste- 
 rior teeth gradually become broad and form a regular pavement on the 
 surface of the jaws similar to that seen in many rays. This structure 
 exists in consonance with the shellfish-feeding habits of the animal, in 
 the exercise of which great crushing and comminuting power is required 
 to be exerted. These fishes are of especial interest, inasmuch as they are 
 the only living representatives of an extensive and widely-distributed 
 group which appeared on the earth far back in the Devonian Epoch, 
 and whose remains, as Owen justly remarks, " would have been scarcely 
 intelligible to us unless the key to their nature had been aiforded by the 
 teeth and spines of the living cestracionts." 
 
 The teeth of the anterior part of the jaw (Fig. 192) are the smallest, 
 and present a compressed conical form with the apex produced into a 
 sharp point. Proceeding backward, they gi-adually assume an oblong 
 oval outline, jirrigressively increasing in size, their sides Ijecoming 
 applied to each other in such a manner as to form a regular pavement. 
 The maximum size is attained at about the fourth tooth from the poste- 
 rior end of the series, after which they decrease rapidly, although still 
 preserving their modified crushing form. 
 
 The progressive changes in size and form, as well as the disposition, 
 of the most highly modified teeth in this animal, are seen to be in direct 
 accord with the uses to which they are put, and serve to illustrate, as so 
 
TEETH OF THE VERTEBBATA. 
 
 373 
 
 many other dentitions do, the reasonableness of the view originally pro- 
 posed by J. A. Ryder, to the effect that mechanical causes have been 
 largely instrumental in bringing about the modifications of the teeth. 
 It will be readily understood that the greatest mechanical advantage 
 would be gained and the greatest pressure exerted by passing the mor- 
 sel to be crushed to the posterior part of the mouth. The teeth in this 
 situation or in its vicinity have sustained the greatest amount of strain, 
 and are consequently most modified, while those of the anterior part of 
 the mouth have been largely exempt from such influences, and are there- 
 fore little modified. I will have occasion to recur to this hypothesis on 
 a future page. 
 
 The teeth of the rays present quite as great, if not a greater, range of 
 variety than do the sharks. In general, they are more numerous, more 
 closely crowded together, and possess forms better adapted for crushing 
 than for seizing and lacerating. They are developed in the same way as 
 in sharks, rising up from the bottom of a thecal fold on the inner sur- 
 face of the jaw and being carried upward by a rotation outward of the 
 
 Fig. 192. 
 
 Lower Jaw of Port Jacksoa Shark {Cestracion phiUippsi). 
 
 membrane in which they are imbedded. In Raia stelluata, from the 
 California coast, the teeth succeed one another vertically, as in Lamna 
 among the sharks, and do not form a close pavement on the biting sur- 
 face of the jaws, they being separated from each other by slight intervals. 
 In form the base of the crown represents an equilateral triangle, with 
 the apex directed forward ; from this a prominent ridge passes back- 
 ward across the middle line of the base, and is produced into a sharp 
 conical point. The teeth of the anterior part of the mouth are the 
 largest, and gradually decrease in size as the canthus or angle of the 
 mouth is reached. In the " barndoor skate " {Raia Icevis) the teeth 
 
374 
 
 DENTAL ANATOMY. 
 
 are more closely set, but are not in absolute contact ; as in Raja stelluata, 
 those of the several rows are arranged vertically, but their bases are 
 more rounded, with only a faint indication of the backwardly project- 
 ing cusp, which is confined to the teeth of the anterior part of the jaws. 
 In the common "stingray" (Trygon centrums) the teeth are some- 
 what quadrangular, and have their sides directly applied to each other, 
 forming a dental sheath of continuous pavement over the working sur- 
 face of the jaws ; those of the successive rows are disposed diagonally. 
 Their crowns are of an oval form, well adapted for crushing and grind- 
 ing hard substances. The "eagle rays" or "sea-devils" present a series 
 of modifications of the teeth which diverges from that of the stingrays, 
 and terminates in the most unique of all dentitions to be found amongst 
 the Vertebrata — viz. that of Aetobatis. Of this group the genus JRhi- 
 noptera possesses tessellated teeth with flat hexagonal crowns, of which 
 
 Fig. 193. 
 
 h C 
 
 Teeth of Rays: a, 6, Rhinoptera; c, Myliohaiis; d, Aetobatis. 
 
 the median or anterior ones may be elongated transversely. The fossil 
 species, R. Woodwardi, has the three median vertical ro\\'S enlarged. 
 In Myliobatis there is only one large median ro\\', with three smaller 
 ones upon either side, while in Aetobatis the teeth of the median row 
 alone remain, and are articulated to each other by a finely serrate border. 
 These modifications are well shown in the accompanying figures. The 
 anomalous sawfisli (Pr/.s^/.s), although in no A\ay peculiar as far as the 
 teeth of the mouth are concerned, nevertheless possesi^es a remarkablv 
 elongated snout, armed upon either side by a roA\- of hard, conical bodies 
 usually referred to as teeth. In their histological structure they agree 
 with true teeth, but exhibit the peculiarity of being lodged in separate 
 sockets and growing from persistent pulps — a condition unusual among 
 fishes. It is more than probable that they are dermal spines specially 
 developed in this situation for some important purpose which is not at 
 present fully determined. 
 
TEETH OF THE VERTEBRATA. 375 
 
 I proceed next to consider the teeth of the Chimserse (Hohoephali), 
 which group some authors make equal in rank with the Elasmobranchii, 
 which then include the sharks and rays only. The peculiarities of the 
 dental succession alone of this latter' group, it appears to me, is quite 
 sufficient to separate them widely from all others, and it seems some- 
 what remarkable that this character has never been utilized by the 
 systematists in their schemes of classification. 
 
 The teeth of the "ratfish" {Chimcera pi imibea) are six in number, of 
 wliich two belong to the lower and four to the upper jaw. The two 
 inferior ones may be described as broad, slightly-curved plates of mod- 
 erate thickness in the form of a right-angled triangle. That border 
 which corresponds to the perpendicular is almost straight, and is lodged 
 in a shallow groove which runs lengthwise along the inner surface of 
 the jaw ; that Avhich represents the base is applied to the corresponding 
 surface of the opposite tooth ; while the border representing the hypoth- 
 enuse forms the free cutting edge of the tooth. This border is some- 
 what devious, being interrupted by three prominences. The inner 
 surface is also slightly ribbed. The two posterior upper teeth are 
 similar plates of a quadrilateral form with their free edges roughly 
 serrate. The two anterior teeth above somewhat resemble ordinary 
 mammalian incisors, and are large and scalpriform. This peculiarity 
 has given them the name "rabbit-fish" or "ratfish." Each tooth 
 has a cavity in the edge by which it is attached and in which the pulp 
 is lodged. But a single set of teeth are developed during the life of the 
 individual, and these are of persistent growth. Another living allied 
 genus, Callor-hynchus, is found in Australian seas, in which the teeth are 
 similar to those of Chimsera, but in the two fossil genera, Edaphadon 
 and Passalodon, supposed to belong to this group, the teeth are anky- 
 losed to the jaw, which is more or less bony. On this account it is 
 more than probable that they are to be referred to the dipnoans rather 
 than to the chimseroids. 
 
 The Teeth in True Fishes. — The teeth of the class Pisces, although 
 apparently presenting an extensive range of modification, have not, 
 debarring the dipnoan ganoids and the plectognath teleosts, as a general 
 rule, departed very widely from the simple conical pattern. There are 
 some forms, however, in which the structure and arrangement are quite 
 anomalous. It is in this group that the maximum development, as far 
 as numbers is concerned, is readied. The salmon, pike, and some per- 
 coids may be cited in which teeth are developed in almost every con- 
 ceivable part of the mouth and number many thousands ; while in 
 others, as the carps and suckers, they are few and confined to the 
 pharyngeal bones. In others, again, as the pipefishes and sea-horses, 
 teeth are entirely absent. 
 
 The teeth of the dipnoans are unique among fishes, and, like those of 
 the chimseroids, are limited in number and grow from persistent pulps. 
 The teeth of the dipnoans, the dental plates of the chimseroids, and the 
 so-called " rostral teeth " of the sawfish are the only examples so far 
 known of permanent teeth to be met with among piscine forms. 
 
 The dental armature of Oeratodus Fosteri (Fig. 194), which may be 
 taken as illustrative of this peculiar group, has six teeth, of which four 
 
376 
 
 DENTAL ANATOMY. 
 
 Fro. 194. belong to the upper and two to the lower jaws. 
 
 Those of the upper series are sujjported upon the 
 palato-quadrate arches and upon a cartilaginous 
 plate which corresponds in position -with the 
 vomer. Those of the lower series are set upon 
 the inner piece of each dentary bone, and be- 
 come firmly attached thereto by ankylosis. 
 
 The two most anterior, and by far the smallest 
 of the upper pairs of teeth, form cutting plates 
 which resemble somewhat the crown of a broad 
 incisor with the posterior border Avell rounded 
 off. They are arranged in the form of a V, with 
 the point directed forward, having their greatest 
 extent in an anterio-posterior direction. After 
 a considerable interval the two large peculiarly 
 constructed upper back teeth appear, similarly 
 placed in the form of a V, and co-ossified 
 with the bony arches. Their greatest length is 
 quite equal to one-third the entire length of the 
 skull, the breadth being much less. Each of 
 these dental plates — for such they may be prop- 
 erly called — has a slightly curved internal bor- 
 der, M'ith the convexity directed inward. The 
 
 'm. 
 
 
 Cei-atoiliis : a, «, teeth of same. 
 
TEETH OF THE VERTEBRATA. 377 
 
 lower or working face presents internally a considerable flat pitted surface 
 reaching the entire length of the tooth, whose plane is directed outward 
 and a little upward. It is slightly broader in front than behind. The 
 outer border is indented by five wide vertical grooves, forming six verti- 
 cally convex lanielliform projections, which encroach somewhat upon the 
 flat surface internal to them as well-defined parallel ridges ; the anterior 
 one passes entirely across the face of the tooth, or, rather, skirts its anterior 
 margin, and becomes continuous with the slightly elevated internal bor- 
 der in this situation. The grooves are deeper in front than behind, leav- 
 ing the anterior projections the most pronounced, while the last one is 
 scarcely perceptible. The points of the projections describe a gentle 
 curve from before backward, the convexity being outward. 
 
 The two teeth of the lower jaw are very similar to those of the 
 upper, but are somewhat narrower, owing to a decrease in width of the 
 flat- pitted surface. As already stated, they are ankylosed to two plates 
 of bone which cover the inner and half of the lower surface of Meckel's 
 cartilage or the central axis of the lower jaw. It will thus be seen that 
 each ramus of the mandible consists of an inner and an outer bony plate 
 enclosing the central cartilaginous axis, which meet in the median line 
 below. To these is added a third piece at the symphysis on its lower 
 surface, so that the bony part of the jaw — which in all probability corre- 
 sponds with the single dentary piece in other fishes — is made up of 
 three, one of which bears teeth. This fact is of great morjihological 
 significance, and will be referred to again when we come to discuss the 
 attachment of the teeth. 
 
 In the two allied genera Protopterus and Lepidosiren the teeth are 
 very similar to those of Geratodus, but in numerous extinct forms refer- 
 red to the dipnoans a considerable amount of variety exists. As regards 
 the development of the teeth in this group, very little is known, and 
 until this has been studied it will be impossible to say whether the den- 
 tal plates are moulded upon a single papilla or represent the combined 
 calcification of several. Judging from their complexity, the latter 
 would seem to be the case. 
 
 The teeth of the remaining ganoids are of the ordinary conical form 
 which prevails to so great an extent in the Teleostei. In number, posi- 
 tion, replacement, etc. they likewise agree so closely with the average 
 teleost dentition that it is unnecessary to make any further mention of 
 them. 
 
 Among teleosts, however, there are several well-marked modifications 
 in the dental armature which deserve to be noticed. One of these is 
 presented by the Plectognathi (plaited jaw), of which the "trigger-fish" 
 (Batistes vetulus) and the " swell toad " (Diodon geometricus) represent 
 the extremes in dentition. The teeth of the mouth of Balistes are 
 twenty-two in number, of which fourteen belong to the upper and 
 eight to the lower jaw. Those of the superior series are disposed in 
 two rows, one placed immediately behind the other, and both are lodged 
 in the premaxillary bones. The most anterior of these rows contains 
 eight teeth, while in the posterior one there are only six. In the front 
 row the mesial pair are the largest, of a subtrihedral form, tapering 
 gradually to an obtuse point. Those upon either side decrease regu- 
 
378 DENTAL ANATOMY. 
 
 larly in size toward the back part of the jaw, and have notched cutting 
 extremities. 
 
 The teeth of the posterior row are applied closely to those of the 
 front row, and are completely concealed when the mouth is closed. 
 They ha\-e a broader, more incisiform pattern. The teeth of the lower 
 series are like the corresponding ones of the upjier front row, and are 
 lodged in the dentary bone. They are attached by slight ankyloses to 
 the respective bones upon which they are su^jported by having their 
 bases placed in a shallow alveolar depression, in the middle of which 
 a conical process of bone rises up and is received into the hollow basal 
 portion of the tooth. The successors of those in use are developed deep 
 down in the substance of the jaws, in bony crypts ^\•hich communicate 
 with the exterior by means of foramina in the side of the jaws in the 
 vicinity of the bases of those in use. All the teeth in this species are 
 said by Owen to be covered with enamel. There are likewise small 
 conical hooked teeth developed ujjon the pharyngeals. 
 
 AA'hile the teeth of Balistes are more nearly affiliated with the normal 
 telec)st condition, those of Diodon, on the other hand, show a much 
 wider departure. ^Vhen the mouth is closed the biting surface of the 
 jaws seems to be invested with a continuous covering of tooth-sub- 
 stance ; upon close inspection this is found to consist of a number of 
 dentine plates closely incorporated ^vith the bone of the jaws and more 
 or less fused together at the base. Each one, however, develops sepa- 
 rately, and takes its place when its predecessor has disappeared through 
 wear. Just inside the margin of each jaw, in the middle line, is to be 
 seen a broad rounded mass consisting of transverse plates of dentine 
 intimately blended and ankylosed to the jaw bones. A faint median 
 longitudinal suture divides each into Uvo parts ; when this becomes 
 more distinct and extends to the edge of the jaw, as it does in some 
 species, it constitutes the mark of generic distinction of the genus Tetro- 
 don. The plates comj)osing this mass, which is peculiar to these fishes, 
 are developed in the same manner as the teeth, and are strictly homol- 
 ogous Avith them. 
 
 Many other examples quite as peculiar as the last one could be cited 
 among the dental organs of fishes, but more time and space would be 
 required than is aiforded the present paper. 
 
 The mode of development is essentially the same as that described 
 for the mammal, with the exce])tion that the dental sacculus is generally 
 simple ; the dentine papilla arises from the corium, and the oral epithe- 
 lium dips down to form a cap-like investment, in both of which calci- 
 fication takes place in the manner already des(.-ribed. AA'ith regard to 
 the succession and attachment (if the teeth in this group, as well as in 
 the Batrachia and Reptilia, some preliminary points in the development 
 of the jaw bones must first be noticed. 
 
 In the early stages of embryonic development each jaw is primarily 
 made up of two cartilaginous Ixus which meet in the median line in 
 front. In the ujiper jaw these bars are known as the palato-pterygoid 
 bars, and in the hjwer jaw as ^NEeckel's cartilage. In the elasmobranchs 
 these bars persist, and the teeth are supported by them. As a conse- 
 quence of this condition, as we have already seen, succession in them 
 
TEETH OF THE VERTEBRATA. 379 
 
 takes place by a movement of the fibrous gum, in which the bases of 
 the teeth are imbedded, outward over the curved surface of the jaw. 
 Coincidently, however, witli the ossification of the skeletal axis the 
 osseous bases of the dermal denticles coalesce to form the dentary bones, 
 as has been shown by Hertwig. By reason of the development of this 
 bony envelope of the primitive axis of the jaw, any further movement 
 of these denticles is prohibited, being firmly co-ossified with it. 
 
 During the coalescence of the denticles a portion of the primitive 
 tooth-bearing membrane is enclosed beneath the fused osseous plates, 
 and retains its original formative energy, thereby furnishing a source 
 of supply quite equal to that of the sharks. The denticles whose basal 
 plates form the sides and under portions of the dentary bones disappear, 
 while those on the summit of the jaw are retained as teeth. A confor- 
 mation of this position by evidence other than that afforded by embry- 
 ology is seen in the dentary bones of Ceratodus, in which each one is 
 made up of three or four pieces which have failed to coalesce. 
 
 The attachment of the teeth, therefore, in this group, as we would be 
 led to anticipate, is by ankylosis to the dentary and other bones which 
 support them. Still, there are many brush-like structures which are 
 identical with true teeth to be found in the mouths of many fishes, 
 which remain imbedded in the lining membrane and do not develop 
 any connection with the underlying bones. There are several ways by 
 which the teeth become fixed to the jaw bones in ankylosis, but the most 
 common is for the central axis of the tooth to be occupied by a cone of 
 osteo-dentine, which blends with the bone of the jaw. Several families 
 of fishes have some of the teeth attached by an elastic hinge, by which 
 they can be bent down in one direction and resume an erect attitude.' 
 
 Teeth of Batbachia and Reptilia. 
 
 As we pass from the dental organs of the more typical fishes to those 
 of frogs, salamanders, newts, etc., constituting the batrachian subdivis- 
 ion, a marked diminution in the number of individual teeth is to be 
 observed. With the appearance of perfected air-breathing organs the 
 complex hyo-branchial skeleton, typical of the fishes, becomes greatly 
 reduced and simplified as the higher forms are approached ; conse- 
 quently, the branchial and pharyngeal teeth disappear in all the 
 Vertebrata above fishes. 
 
 In all those Batrachia in which teeth exist they are usually disposed 
 in a single row on the borders of the jaws, and are supported by the 
 maxillary, premaxillary, and dentary bones respectively. In addition 
 to these, each vomer (for there are two) bears a single row of teeth, 
 between which and the maxillary row the lower jaw bites. As a rule, 
 the Reptilia, on the other hand, lack the vomerine set, but in some of 
 them (serpents, for example) teeth are developed upon the pterygoids 
 and palatines, as well as upon the maxillary and dentary bones. 
 
 The teeth of the Batrachia present so limited a range of variation that 
 the description of one will serve to give a general idea of the dentition 
 of the whole group. 
 
 ' This fact was first noticed by Prof. Gill. 
 
380 DENTAL ANATOMY. 
 
 It should be stated here that some of the tailless species (toads) are 
 quite edentulous, while in others (the frogs) teeth are absent in the lower 
 jaw. All the existing tailed batraehians, however, are provided with 
 teeth which present practically the same pattern and disposition which 
 obtains throughout the entire sub-class. 
 
 An excellent and easily-obtained example of this latter subdivision is 
 found in the Alleghany Menopoma, popularly known as the "hell- 
 bender." In this animal the skull is remarkable for its flatness and 
 breadth, as well as the almost perfect semicircular outline which the 
 dentigerous surface of the jaws presents. The mandible, as in the fishes, 
 is composed of angular, articular, and dentary pieces, and is suspended 
 to the cranium by means of two bones known as the squamosals through 
 the intervention of the quadrates. 
 
 The palato-quadrate or palato-pterygoid arch is not so well defined, 
 although the principal elements are present. The vomers are two in 
 number, and occupy their usual position behind the maxillaries, sharing 
 in the formation of the bony roof of the mouth. The maxillaries and 
 premaxillaries also have the same position as in the fishes, but are less 
 mobile, on account of sutural connections with the surrounding bones. 
 
 The biting surface of each jaw is produced into a sharp ridge l)y rea- 
 son of the existence of a well-marked ledge extending the full length of 
 its internal face. This ledge is converted into a groove in the recent 
 state by a fold or flap of the gum, which forms its internal wall, and is 
 in all probability homologous with a similar structure (the thecal fold) 
 found in the sharks. At the bottom of this groove the tooth-germs of 
 the successive sets of teeth are developed. It will be seen, therefore, 
 that the general arrangement is not different from that of the sharks ; 
 but this important difference is to be observed : in the sharks the bases 
 of the teeth are at first directed U23i,\-ard, and it is only when they are 
 about ready to take position on the working surface of the jaw that they 
 assume the erect attitude ; this, as we have already seen, is due to the 
 movement of the entire gum outward. This manner of replacement 
 gives weight to the conclusion that the teeth in the sharks are invagi- 
 nated dermal spines, the position of which we would expect to find 
 reversed upon the inside of the jaws. 
 
 In the batraehians, on the contrary, the teeth are said to have an erect 
 position from the earliest stages of development, and it is less easy to 
 see how they represent dermal spines or how the position came to be 
 reversed. Believing, however, that all the maxillary and mandibular 
 teeth were originally of tegumentary origin, as is clearly demonstrable 
 in the sharks, it is more than probable that the arrest of the outward 
 movement of the gum in the batrachian by the aj^pearance of ossifi- 
 cations around Meckel's cartilage to form the dentary bones is responsi- 
 ble for this change. It is quite possible that the tooth-germs of the 
 batrachian do at first have the same position as those of sharks — that is 
 to say, with the points directed downward — and that the formative 
 energy of the tissues beneath causes them to become erect at a compara- 
 tively early period. 
 
 We have already stated, in connection with the account of the dental 
 organs of the elasmobranchs,that tooth-succession is primarily due to 
 
TEETH OF THE VEBTEBBATA. 381 
 
 the evagination of the lining membrane of the oral cavity. Whether 
 this is caused by unequal rate of growth of the surrounding parts, 
 whereby this lining membrane is forcibly pulled outward and replaced 
 from within, or whether the formative energy inherent in the membrane 
 itself causes it, is difficult to determine. This primary cause of succes- 
 sion is profoundly affected by the development of an osseous sheath 
 from this same membrane around the central cartilaginous axis of the 
 jaws. It seems plausible that secondarily the cause of tooth-succession 
 is to be sought for in the proliferation of the cellular elements beneath 
 the young and growing germ. 
 
 To the inner side of the external osseous wall of this groove in Meno- 
 poma the functional teeth are attached ; their bases are slightly enlarged 
 and extend quite to the bottom of the groove, while the tapering crowns 
 reach considerably above the level of the jaw. Attachment takes place 
 by the ankylosis of that part of the base which is in contact with the 
 outer wall to the bone of the jaw through the intermediation of osteo- 
 dentine. This manner of implantation is known as " pleurodont," on 
 account of the fancied resemblance of the teeth so attached to ribs. 
 
 The teeth in use at any given time are from thirty to forty in num- 
 ber upon either side in each jaw ; they are subequal in size, and are 
 placed with great regularity, being separated by spaces about equal to 
 the width of a single tooth. Their crowns are sharp-pointed and 
 slightly recurved ; they are said in some species to be tipped with 
 enamel, which is probably true of all. The vomerine teeth are fewer 
 in number than the maxillary, there being not more than twelve or 
 fifteen upon either side. Their line of direction and manner of implan- 
 tation coincide with the maxillary row external to them, agreeing with 
 those also as to size and form. 
 
 In some of the extinct batrachians, notably the labyrinthodonts, there 
 were several teeth of the maxillary and premaxillaiy set considerably 
 enlarged and of a caniniform pattern. The species of this section were 
 mostly of large size and presented a formidable dental armature. They 
 likewise differ from all other of the Batrachia in that the teeth were 
 implanted in distinct sockets, and were rarely if ever attached to the 
 body of the jaw by ankylosis. The structure of the teeth is a curiously 
 complex one, and finds no parallel throughout the entire A^ertebrata, 
 save in one extinct saurian (lehthyosawus) and several fishes, which 
 exhibit a similar condition in a less perfected degree. 
 
 The external surface of the crowns of the teeth in the labyrintho- 
 donts is marked by a number of longitudinal ridges, separated by what 
 at first sight would appear to be comparatively shallow grooves extend- 
 ing from the base to the apex of the crown. Upon cross-section, how- 
 ever, these fissures are seen to penetrate into the body of the tooth to a 
 remarkable depth — ^to a point, in fact, quite near the pulp-cavity or cen- 
 tral axis of the tooth, where they are separated from it by a thin wall 
 of dentine. The entire outer surface is covered by a thin layer of 
 cement, which is reflected inward to the bottoms or internal termina- 
 tions of the fissures just mentioned. The cut edges of this reflected 
 layer of cement, which is of uniform thickness throughout, are almost 
 straight for a short distance beneath the surface, but soon become very 
 
382 DE2iTAL ANATOMY. 
 
 tortuous. The arrangement of the dentine is as follows : The axial 
 portion of the tooth consists of a central cone of dentine hollowed out 
 in the centre to receive the pulp. In cross-section this cone appears as 
 a ring surrounding the pulp-cavity ; from it plates of dentine, -which are 
 cleft bv the fissures from without, radiate to the periphery, pursuing the 
 same tortuous course as that of the fissures. These dentinal plates are 
 separated from each other by fissures which radiate from the axial cav- 
 ity, but do not reach the exterior of the crown. Some of the dentinal 
 plates do not arise from the central ring, but apjjear on transverse sec- 
 tion as processes fi'om the periphery of the crown directed toward the 
 central axis, thus causing the fissures which radiate from the pulp-cavity 
 to become bifurcated at their outer or peripheral tei'minations. Some 
 of these accessory processes reach but a short distance toward the inte- 
 rior, while others penetrate halfway or more to the centre. 
 
 This complexity in the arrangement of the tooth-substances has sug- 
 gested the name of the typical genus, Labyrinthodon, which was orig- 
 inally described by the great anatomist Prof. Owen. Just how it has 
 been produced is difficult to understand. 
 
 Teeth of the Reptilia. — This class of vertebrated animals in- 
 cludes snakes, lizards, crocodiles, turtles, etc., and, considering the 
 extinct as A\ell as the recent forms, is divisible into eleven distinct 
 orders, according to Prof. Cope's classification. Of these, but five are 
 represented in the existing fauna, the others having become extinct in 
 the difi^rent epochs of the earth's history. 
 
 The batrachians make the nearest approach to the permanent gill- 
 breathing vertebrates in all the essential features of their structure ; the 
 Reptilia, on the other hand, furnish us with the transitional forms lead- 
 ing to the avian and mammalian stems. It is a very significant fact — 
 and one upon which the doctrine of evolution is primarily based, so far 
 at least as the Vertebrata are concerned — that the lowest forms appeared 
 fii'st in the order of time, and were followed by those higher in the 
 organic scale ; thus we have the cartilaginous fishes as the earliest rep- 
 resentatives of vertebrated animals ; after them come the batrachians ; 
 next the reptiles and birds, and finally mammals. It must be borne 
 in mind, however, that the higliest of one group is not always most 
 nearly related to the group next above it ; for example, if we compare 
 the structure of a bony fish M'ith that of a salamander, a great interval 
 will be found to exist, but if we institute a comparison between the latter 
 and a dipnoan fish, which is comparatively little removed from the car- 
 tilaginous forms, this interval will be found to be materially diminished. 
 Thus, the conclusion is obvious that the Batrachia s})rang not from the 
 higlier bony fish, but from some generalized representative of the pis- 
 cine type. The same reasoning can be applied to other divisions. 
 
 As regards the Reptilia, the chief distinction between them and the 
 Batrachia consists in the circumstance that the latter during the larval 
 stages of their existence breathe by means of gills like the fishes, where- 
 as tlie Reptilia breathe by means of true lungs from the time of birth. 
 Iniportant osteological differences are found in the bones of the skull. 
 
 The earliest appearance of the Reptilia dates back to the Permian 
 Epoch, where they are represented by a group of peculiar batracbian- 
 
TEETH OF THE VEBTEBBATA. 383 
 
 like reptiles which has been designated the Theromorpha by Prof. Cope. 
 This group includes two important divisions, one of which, the Anonio- 
 dontia, was first described by Prof. Owen from the Triassic (?) deposits 
 of South Africa ; the other is the Pelycosauria, which is so far known 
 only from the American Permian. 
 
 The osteological structure of this order furnishes us many transitional 
 characters between the Batrachia and more typical Reptilia, on the one 
 hand, while on the other they seem to stand in ancestral relationship to 
 the prototherian Mammalia. Their batrachian affinities are manifested 
 in the structure of the pectoral and pelvic arches, in the structure of the 
 limbs, and the possession of teeth on the vomer. In the absence of a 
 parasphenoid bone in the base of the cranium and the unicondylian 
 condition of the skull they are markedly reptilian. The structure 
 of the pelvic and pectoral arches and limbs, together with the intercen- 
 tral articulation of the ribs, allies them with the lower Mammalia. 
 
 Their dental organs present a considerable variety of structure — in 
 some instances departing widely from the simple conical form usual 
 among the other orders of this class of the Vertebrata. In one genus 
 (Dlmetrodon) there were two large caniniform teeth in each premaxil- 
 lary, implanted in distinct sockets ; these were followed by a single row 
 of maxillary teeth, whose crowns resemble somewhat the premolars 
 of the dog in general pattern. They were lodged in distinct alveoli, 
 and exhibit the remarkable peculiarity of being implanted by double 
 fangs, or rather single ones deeply grooved upon either side, otherwise 
 unknown among the Reptilia. The first tooth behind the maxillo-pre- 
 maxillary suture is enlarged into a canine, and the entire maxillary 
 series does not exceed fifteen in number. The palato-pterygoid arch is 
 present, and one of its elements, probably the pterygoid, is thickly 
 studded with small conical teeth irregularly disposed. Another element 
 which lies internal to this last-mentioned bone is described by Prof. 
 Cope as bearing a single row of teeth. Other genera related to this 
 one are Theroplewa, Glepsydrops, etc. of Cope, which present minor 
 differences in the form and size of the corresponding teeth. 
 
 A nearly-allied family of this group is the Diadectidce, likewise 
 described by Cope. A typical example of the dentition of this family 
 is seen in Empedocles molaris, wherein the pattern of the crowns of the 
 molars is thoroughly unique. The teeth are disposed on the borders of 
 the premaxillary, maxillary, and dentary bones, as well as upon the 
 vomer, which forms a median keel in the roof of the mouth. The max- 
 illo-premaxillary set in the upper jaw describe a sigmoid curve in their 
 line of implantation, and form an uninterrupted series from the front 
 to the back of the mouth. There are fourteen teeth belonging to this 
 series, of which the first two are larger than those immediately succeed- 
 ing them. They have obtuse subconic crowns, and are lodged in dis- 
 tinct alveoli. From this point the teeth gradually decrease in size up 
 to the sixth, when they again become larger and more complex in pat- 
 tern. The crowns of the typical molars have a much greater transverse 
 than longitudinal extent ; the grinding surface is somewhat elliptical in 
 outline, and is provided with a submedian cusp which stands nearer the 
 outer than the inner border. The portion of the crown external and 
 
384 DENTAL ANATOMY. 
 
 internal to the median cusp is horizontal, and has its surface thrown 
 into conspicuous folds or wrinkles. The teeth of the lower jaw are 
 essentially like those of the upper. The vomerine teeth are small and 
 conical, and arc disposed in two longitudinal rows. 
 
 In the tyjjical genus (Diadedes) there is a well-developed canine, 
 ^vhile in another member of tlie family {Helodedes) there are canines 
 and a double row of maxillary teeth upon either side in the dentigerous 
 surface of these bones. 
 
 In the other subdivision of this order — viz. Anmiodontia — the den- 
 tition is reduced to lai-ge pointed, recurved tusks, which are lodged by 
 distinct sockets in the maxillary bones. The rest of the jaw is edentu- 
 lous, and was in all probability ensheathed in a corneous substance, as 
 in the existing turtles. Other extinct members of this subdivision, 
 notably Bhi/nchosaiiriis and Oudenodon, were entirely edentulous, and 
 in all probability were the ancestors of the turtles. 
 
 That division known as the Crocodilia includes the alligators, croco- 
 diles, gavials, etc., \vhich are separated from the other Reptilia by a 
 number of important osteological characters, prominent among ^hich is 
 the complete development of the bony roof of the mouth. Teeth are 
 supported by the premaxillar)'^, maxillary, and dentary bones only, the 
 palatines and pterygoids having approximately the same relations and 
 edentulous condition as in the mammalian skull. In no crocodilian so 
 far known are the teeth ever ankylosed to the body of the bones upon 
 AA'hich they are borne, but, on the contrary, they are set in distinct 
 sockets disposed in a single row along the margins of the tooth-bearing 
 bones. In young specimens the alveoli are apt to be ill defined, more 
 especially toward the back part of the jaws, but as age advances the 
 bony partitions become more distinct. On account of each tooth having 
 a distinct alveolus, this division of the Reptilia was formerly known as 
 the thecodonts, in C()ntradistinction to the pleurodonts — a condition 
 already mentioned in connection with Menopoma — and acrodonts, pres- 
 ently to be described. 
 
 A good example of the dentitifin of a crocodilian reptile is afforded 
 by the Mississippi alligator (Alligator jl//.ss/.'>-.s'()jy>(V'«.s/.s), A\'liich can be 
 found in almost any osteological collection in this country. In the 
 upper jaw there are from eighteen to twenty-two teeth upon either side, 
 of which five are usually set in eacli jjremaxillary and the remainder in 
 the maxillary bones. The most anterior of the premaxillarv series is 
 the smallest, from which they gradually increase in size to the fourth, 
 Avhieh is nearly twice as large as any of the others; the fifth is about 
 equal to the third. The first of tlie )uaxillary series is likewise the 
 smallest ; the three succeeding teeth gradually increase in size until the 
 third is reached (the ninth ci.iunting from the first tooth in the premax- 
 illary), ^vhich is known as the canine of the upper jaw. The eighth and 
 tenth are frequently as large as the canine. Behind, the teeth become 
 smaller, and are again enlarged in the vicinity of the sixteenth or sev- 
 enteenth from the first premaxillarv tooth ; from this point they rapidly 
 diminish toward the posterior end of the tooth-line. 
 
 In the lower jaw the teeth are likewise of unequal proportion, but 
 those which are largest in the one series are opposed by the smallest of 
 
TEETH OF THE VERTEBRATA. 385 
 
 the opposite set ; Ihus that tooth which is Ciininiforra in the lower jaw 
 is the fourth, and bites in front of the corresponding tooth above. It is 
 received into a deep fossa in the upper jaw just internal to the alveolar 
 border at the point of junction of the niaxiilarv ^vith the premaxillary 
 bone, or between the fifth and sixth teeth above. It not iinfrequently 
 happens in old specimens that this fossa is converted into a foramen 
 leading to the external surface of the skull by the perforation of its 
 base. In such cases the point of the lower canine passes through the 
 upper jaw and appears upon the upper surface. 
 
 The only important distinction between the alligators and the croco- 
 diles consists in the fact that in the latter this fossa is open externally, 
 causing the tooth-line to be interrupted by a deep notch, whereas in the 
 latter it is intact. 
 
 Both the a,lligators and the more typical crocodilians are remarkable 
 for the breadth of the palate and the flatness of the muzzle, as well as 
 the alternate increase and decrease in the size of the teeth from before 
 backward ; but in the gavials the snout is very long, narrow, and almost 
 cylindrical ; the teeth, too, are more nearly equal and of more regular 
 proportions. 
 
 In the alligator the anterior teeth have conical crowns terminating in 
 sharp points, which are slightly recurved. The posterior ones have 
 more obtuse crowns, which terminate below by a moderately well-defined 
 neck. In some species the anterior and posterior surfaces of the crowns 
 are produced into trenchant edges, which may be more or less serrated ; 
 in the alligator this is but faintly marked. 
 
 The manner of succession is not different from that of the other lower 
 vertebrates. If the root of a tooth in place be exposed, the successional 
 sets in various stages of development will be seen below and to the inside 
 of it, arranged in the form of a nest of crucibles. This arrangement 
 results by reason of the absorjjtion of the inner wall of the root of the 
 tooth in place which the immediate successor causes. By this means the 
 point of its crown comes to occupy the pulp-cavity of the functional tooth. 
 
 In the order LaeertiUa, which includes the lizards proper, a more 
 varied development of the dental organs is met with. As a general 
 rule, teeth are borne upon the pterygoid and palatine as well as upon 
 the maxillary, premaxillary, and mandibular bones. There are, how- 
 ever, some exceptions, one of which is afforded by our little " horned 
 .toads " (Phrynosoma), in which the palatines and pterygoids are eden- 
 tulous. The teeth may be either " pleurodont " or " acrodont " in their 
 manner of implantation, but in certain extinct forms (e. g. MososauruH) 
 both conditions are to be observed. In the case of acrodontism the 
 bases of the teeth are soldered to the summits of slight elevations which 
 arise from the alveolar border of the jaws. Pleurodontism, as has 
 already been mentioned, consists in the ankylosis of the base and outer 
 sides of the teeth to the outer wall and bottom of the dental groove. 
 Another varietv of implantation, known as ccelodontisiii, has been 
 described, in which the tooth has a permanent pulp-cavity, and is 
 attached to the outer wall, leaving the base free; it should be men- 
 tioned that in pleurodonts the pulp-cavity is not permanent; it 
 soon becomes obliterated, leaving the tooth solid. 
 
 Vol. I.— 25 
 
386 DENTAL ANATOMY. 
 
 A fair example of a pleurodont lacertilian is aiforded by the majority 
 of the numerous species of the Iguanidce, although some of the members 
 of the iguanian family, such as Murm, Lophyrus, CMoUs, and others, 
 are acrodont. In the horned iguana {ildopocerus cornutua) the max- 
 illary and premaxillary teeth are from twenty-two to twenty-three in 
 number upon either side. The central ones of the premaxillary set, 
 of which there are four, are smallest, the outer ones slightly enlarged. 
 These, together with the first five or six maxillary teeth, have sub- 
 conic recurved crowns, while the crowns of the posterior maxillary 
 series are laterally compressed into anterior and posterior cutting 
 edges and terminated by a principal cusp. Of the two edges, the 
 anterior is the longer and is interrupted by three minor cusps, the 
 posterior being shorter and bearing only a single accessory cusp. The 
 presence of these cusps gives the crown a serrated appearance ^\-hen 
 viewed from the side. 
 
 The teeth of the kiwer jaw are from twenty to twenty-two in number 
 upon either side, and are similar in form t(j those above, Avith the excep- 
 tion that there are generally Uvo accessory cusps upon either trenchant 
 edge of the cr< iA\'n. There is in addition to these a single row of small 
 conical teeth supported by each pterygoid bone ; the number of the>e 
 varies from five to seven. 
 
 The only lacertilian A\-hich is known to be poisonous is the " Gila 
 monster " (Hclodcnaa ■■^iispednm) of our American fauna. Recent 
 experiments of Drs. Mitchell and Eeichart of Philadelphia have 
 demonstrated beyond doubt the poisonous qualities of its salivary 
 secretion. Considerable interest therefore attaches to its dental organs, 
 as well as to the anatomy of the poison-glands ; this latter subject I 
 am, unfortunately, not in a positiim to describe, and will therefore 
 limit what I have to say here to a consideration of the teeth onlv. 
 
 This animal, of -whicli there are two spec'ics, is confined to the desert 
 A^-astes of the iSouth-'\vestern United States, A\-hei-e it is not of rare occur- 
 rence. In life it has a rather i-epugnant a2-)pearance, "which is no doubt 
 inci-eased liy our knowledge of its poisonous qualities. It attains a 
 length of eighteen inches or two feet, and is covered with bright yellow 
 spots, a circumstance which gives the name Hdoderma to the genus, 
 meaning "sun skin." Its venomous nature was not known until the 
 experiments above mentioned "were made, although Prof Cope had 
 reason to suspect as much, and gave the name "snspectum" to the 
 species, which he descril)e(I several years before. 
 
 The teeth are supported by the premaxillary, maxillary, and dentary 
 bones, the palatine and pterygoids being edentulous. Those of the pre- 
 maxillary, of which there are three upon each side, are the smallest of 
 the upper teeth. Tliey increase regularly in size from before backward, 
 and fijfm a continuous series, with the maxillary teeth behind, ^\\\u\^ 
 continue to augment their dimensions up to the eighth tooth from the 
 median ])remaxillary pair or the fiftli of the maxillary set. From this 
 point backward the two remaining teeth become slightly smaller. The 
 teeth of the lower jaw are nine in number, and are disposed very much 
 in the same manner as those above — tlie smallest in front and the 
 largest toward the back part of the mouth. X considerable disparity 
 
TEETH OF THE VERTEBRATA. 387 
 
 in size exists between the inferior series and the corresponding teeth 
 above, those below being much the longer and more robust. 
 
 In their manner of implantation they cannot be said to be either 
 acrodont or pleurodont, but rather intermediate between the two. The 
 internal aspect of each jaw, which is remarkable for its breadth, is slightly 
 bevelled internally, causing the outer edge to rise a little above the inner. 
 Nearer the outer than the inner edge of this bevelled surface are a num- 
 ber of low bony elevations, corresponding to the number of the teeth in 
 functional use, to the summits of which they are attached by ankylosis. 
 In some instances these elevations are so faintly indicated that the teeth 
 appear to be soldered to the bevelled surface of the jaw directly. Just 
 internal to the basis of the functional teeth may be seen the successive 
 sets in different stages of development. In the recent state they are 
 covered by a fold of the gum, which likewise covers up the bases 
 of the functional teeth. 
 
 The form of the crown is that of a long, slender, sharp-pointed cone 
 curved inward and backward. The anterior surface of each tooth is 
 marked by a well-defined groove extending from the base to the apex. 
 It is somewhat deeper at the base than the summit, and is most distinct 
 in the teeth of the lower jaw. The intervals between the bases of the 
 teeth allow abundant room for the accommodation of poison-glands, the 
 secretion of which is conveyed down these grooves and thus injected 
 into the wound which the teeth inflict upon a prey. 
 
 Another group of curious and interesting reptiles is the Dinosauria, 
 which became extinct at the close of the Cretaceous Epoch. They are 
 of especial interest on account of their remarkable bird-like affinities, 
 and, according to the views of many authors, were the direct progenitors 
 of the struthious birds, or ostriches, emus, etc. They were mostly of 
 gigantic size, and some of them are remarkable for the great number 
 of teeth contained in the upper and lower jaws ; others, again, were 
 almost edentulous. 
 
 In the iguanodonts and hadrosaurs, which are typical representatives 
 of the herbivorous division of this order, the crowns of the teeth are 
 somewhat expanded and are marked externally by vertical ridges, while 
 the internal portion is smooth and rounded. In Iguanodon the external 
 surface, to which the enamel is confined, is traversed by three vertical 
 ridges, separated by vertical grooves ; the anterior and posterior edges 
 were serrated, as in Iguana, before the crown was abraded by wear. 
 In the hadrosaurs there is but one vertical ridge, which is external in the 
 upper and internal in the lower teeth. The part which bears this ridge 
 is known as the enamel or cementum plate. Prof. Cope has recently 
 had the opportunity of satisfactorily determining the dental peculiarities 
 of this group of gigantic saurians, as exemplified by the genus Dieloniiin, 
 through the fortunate discovery of an almost complete skeleton by Dr. 
 Russel Hill and the author in the Bad Lands of Dakota during the 
 summer of 1882. 
 
 According to Prof. Cope's description, there are in all two thousand 
 and seventy-two teeth. Of these, there were not more than two or three 
 hundred in use at one time, the others being arranged in successive rows 
 beneath, ready to take the place of the functional ones when they were 
 
388 DENTAL ANATOMY. 
 
 worn out. One striking peculiarity which this reptile presents is in the 
 dentigerous character of the ^jJcnidl and the edentulous condition of 
 the dentary bones of the mandible. The teeth are relatively small, and 
 are placed at some distance from the anterior part of the mouth. This 
 part of the ja-\vs is believed to have been occupied l)y a kind of horny 
 sheath similar to that found in birds and turtles. 
 
 The proportions of the limbs were those of the kangaroo, the p(jsterior 
 greatly exceeding the anterior in size. The general shape of the skull 
 is very much like that of a bird ^ith a large spatulate Ijeak ; it Avas 
 supported upon a long, flexible neck, Mhich was doubtless u,-eful to the 
 animal in gathering the soft aquatic vegetation upon A\-hic]i, from the 
 character of its teeth, it is supposed to have subsisted. It likewise 
 had a powerful tail, much deeper than thick, A\hich probably served 
 not only as a fifth limb in balancing the weight of the animal, but 
 could also have been useful as a swimming organ. The feet were pro- 
 vided with true hoofs. 
 
 The carnivorous dinosaurs were scart-ely inferior in size to the her- 
 bivorous sijecies, but were of a more slender and active build. Their 
 jaws were provided -with large, powerful conical teeth, better adapted 
 for the capture of living animal pre}'. The terminal phalanges were 
 ensheathed in distinct claws. 
 
 Another order of the Reptilia, and one which is probably best known, 
 is the Ophidia, or snakes. Especial interest attaches itself to the dental 
 organs of many of this group, inasmuch as their jjoisonous bite consti- 
 tutes one of their most conspicuous features and renders them particu- 
 larly obnoxious as well as dangerous to life. 
 
 According to most systematists, the order is divisible into fi\-e sub- 
 orders, which have been defined as fbllo\\> : 
 
 I. " The palatine bones widely separated, and tlieir long axes longitndinal ; a trans- 
 verse (ecto-pterygoid) bone; tlie pterygoids unite with the quadrate bones." 
 (I. "None of the maxillary teeth grooved or canaliculated " . . Asinca. 
 
 Ii. "vSome of the posterior maxillary teelh grooved" Tortricina. 
 
 '-. "Grooved anterior maxillary teeth succeeded by solid teeth" Proieroc/lyphia. 
 (1. " Maxillary teeth few, canalicnlated, and fang-like" . Soleiioc/li/pliia. 
 
 II. " Tlie palatine bones meet or nearly meet in the base of the skull, and their long 
 axes are transverse. Xo ecto-pterygoid bone; the pterygoids are not con- 
 nected with the quadrate bones" (Huxley) Scalecophidia. 
 
 The first of these sub-orders includes nearly all of the harmless or non- 
 venoniotis species, of Avhicli the black snake, garter snake, boa, etc. are 
 familiar examples. The second includes a single family with few species, 
 said to be harmless ; they are confined to Africa. The third sub-order 
 embraces such forms as the deadly cobra, the C'oral snake, harlequin 
 snake, and others. The fourth includes the vipers, rattlesnakes, adders, 
 etc. The last is represented by few species which are non-venomous. 
 
 In general, the dentigi.Tous elements of the o])hidian skull may be said 
 to consist of maxillary, palatine, and pterygoid ))ones of the upper and 
 the dentary bones of the lower jaw, although in the pythons and tor- 
 trices teeth exist upon the premaxillaries as well. In Rachiodon, a 
 singular African species of the Asinea, the teeth of the ja\vs are 
 extremely small and soon disappear. This loss is compensated tor by 
 an excessive development of the hypopophyses of several of the anterior 
 
TEETH OF THE VERTEBRATA. 389 
 
 vertebrse, which pierce the superior wall of the oesophagus and are 
 tipped with a layer of liard cementum. The food of this species con- 
 sists of the eggs of small birds, which it swallows \\holc. During the 
 act of deglutition the t'ulcareous shell is brought into contact \vith and 
 crushed by these oesophageal teeth, thus preventing the escape of any of 
 the nutritious substances. 
 
 In the non-venomous species the maxillary bone is long, and bears a 
 row of teeth which are of variable size in the different parts of the jaw 
 in different genera. In some the teeth are largest in front and smallest 
 behind ; in others it is the reverse of this ; while many have the teeth 
 of equal size throughout; then, again, certain teeth of either jaw may 
 be specially enlarged and separated from the others by a diastema. All 
 these conditions have received distinct names. 
 
 All serpents are acrodont, and the crowns of the teeth consist of long, 
 sharp-pointed, recurved cones which are designed more to prevent the 
 escape of a struggling prey than as instruments of mastication. The 
 two rami of the lower jaw are bound together at the symphysis by 
 elastic ligaments, which, together with the great distensibility of the 
 throat, due to the mobility of the suspensory bones, alloAvs them to 
 swallow objects many times larger than the usual diameter of the body. 
 During the act of swallowing the recurved and pointed leetli act as so 
 many hooks to prevent a backward movement of the object. 
 
 In the sub-order known as the Pvoteroglyphia the maxillary bone is 
 shortened somewhat, and the anterior teeth are enlarged and grooved on 
 their anterior faces. One of these teeth (the anterior) is the largest, and 
 is denomi'nated the fang. It is permanently erect in these serpents, being 
 ankylosed to the maxillary bone, which is capable of comparatively little 
 movement. 
 
 In the solenoglyphs, on the other hand, of which the rattlesnake is an 
 excellent example, the maxillary bone attains its maximum of abbrevia- 
 tion and supports a single tooth, the fang.' It is movably articulated 
 with the lachrymal above by means of a ginglymoid joint. The fang is 
 canaliculated or perforated in the direction of its long axis by a canal 
 which opens near its point. This canal results from the fusion of the 
 free edges of the anterior groove, which remains open in the fangs of the 
 proteroglyphs. When the mouth is closed, the maxillary bones are 
 retracted and the fangs lie parallel with the roof of the mouth ; when 
 the animal " strikes," the maxillary bones are extended by special mus- 
 cles and the fangs become erect. 
 
 The canal of the fang receives at its proximal termination the duct 
 of the large poison-gland, which lies above it, so that when the punc- 
 tured wound is inflicted the poisonous secretion is injected into it. This 
 is facilitated by a coincident contraction of the muscles which surround 
 the gland. It has been suggested by Owen that as the quantity of saliva 
 and lachrymal secretion is increased during particular emotions, so the 
 rage which stimulates the venom-serpent to use its deadly weapon must 
 
 1 Usually, a number of teeth are found just behind the fang in this bone, some of 
 which are nearly or quite as large as the fang itself. These are the teeth which are 
 destined to succeed the functional fang whenever it shall have been shed or lost by 
 accident. 
 
390 DENTAL ANATOMY. 
 
 be accompanied with an increased secretion and great distension of the 
 poison-glands. 
 
 In reference to the poisonous character of this secretion, it is a well- 
 known fact that the normal saliva of many animals is more or less 
 dangerous when injected directly into the blood, and that in a state of 
 rage it is rendered more so. Prof. Cope has recently called my atten- 
 tion to the possible explanation of the poisonous character of this anal- 
 ogous secretion of the venomous serpents : that since their peculiar 
 method of locomotion ^^•ould expose them most frequently t(j injuries 
 and inconveniences calculated to excite this state, the normal salivary 
 secretions have become accordingly modified. 
 
 The remaining orders of the Reptilia do not exhibit any important 
 modifications of the dental system worthy of special notice. 
 
 THE TEETH OF THE MAMMALIA. 
 
 A^'iTH a consideration of the teeth of the Mammalia a's-c enter upon a 
 study of a series of dental organs whose complexity, variet)', and spe- 
 cialization surpass those of any other group of the Vertebrata. The 
 wide diversity of conditions under which the different members of this 
 great group exist would of itself lead one to anticipate a corresponding 
 diversity in dietetic habits, as well as or'gans suitable for the prehension 
 and assimilation of the substances by which they are nourished. The 
 broad grinding surface afforded by the molar tooth of the elephant, the 
 sharp, trenchant, sectorial dentition of the lion, the great scalpriform 
 incisors of the beaver, the small cylindrical teeth of the armadillo, are 
 a fi'w examples of the great range of variety which mammals exhibit 
 in the form of their dental organs. 
 
 As already remarked in the introductory pages, this study is greatly 
 facilitated by considering it from the standpoint of evolution, or rather 
 in the light of the palseontological history of the group. If T,\'e look 
 upon the fossil remains of any given period of geologic time as the 
 representatives in part of the animals which at that time inhabited the 
 earth, it then becomes i if the utmost importance to ascertain the exact 
 relationship A\'liich the animals of each period bear to those which have 
 preceded and succeeded them in time. It is needless to say that the 
 conclusions which we are com]3ellcd to draw from studies of this cha- 
 racter are important and significant, and serve to bring into the closest 
 connection many isolated facts which if considered by themselves would 
 be wholly unintelligible. 
 
 Some objection to this method of treatment will doubtless be raised 
 by those who do not accept evolution as a demonstrated fact, or those, 
 again, ^vho consider our information concerning extinct forms too meagre 
 for purposes of generalization. In answer to these objections it must be 
 ur,u'cd that palseontological law com]iels us to recognize the important fact 
 that in every department of life the generalized has preceded the spe- 
 
TEETH OF THE VEBTEBBATA. 391 
 
 cialized in time; we pass from the simple to the complex, whether an 
 individual organ or the entire organism be considered ; and the teeth 
 form no exception to this rule. So conohisive is the testimony which 
 it is now possible to adduce in support of this general proposition, and 
 so pregnant are the minds of modern biologists with this belief, that it 
 seems utterly impossible to escape the conviction that life from its earli- 
 est inception has been continuously, and in many instances progressively, 
 modified. As to the nature of the causes which have induced this modi- 
 fication, there is much less unanimity of opinion. It is a question 
 regarding which the most exhaustive philosophic discussion is now in 
 jjrogi'ess. 
 
 When we speak of the origin of mammalian teeth, it is necessary to 
 have some definite knowledge of the origin of this class of animals before 
 we can be absolutely certain of just what constitutes a primitive mamma- 
 lian dentition. Unfortunately, the evidence which would enable us to 
 determine the ancestry of the mammal beyond dispute has not as yet 
 been found, but it appears sufficiently evident that we are limited in our 
 choice to the Batracliia and Reptilia of the Permian Period. Huxley, 
 who has devoted considerable attention to this subject, concludes that 
 we must go backward past the Reptilia directly to the Batrachia. This 
 conclusion is primarily based upon a comparison of the pelvic arch of 
 the monotremes with that of the batracliians. In addition to the evi- 
 dence drawn from this source, upon which his argument is principally 
 founded, the following reasons are given for this vie%v : " The Batrachia 
 are the only air-breathing Vertebrata which, like the jNIammalia, have a 
 dicondylian skull. It is only in them that the articular elements of the 
 mandibular arch remain cartilaginous, while the quadrate articulation 
 remains small, and the squamosal extends down over the osseous ele- 
 ments of the mandible, thus affording an easy transition to the mam- 
 malian condition of those parts. The pectoral arch of the monotremes 
 is as much batrachian as it is reptilian or avian. The carpus and tarsus 
 of all Reptilia and Aves, except the turtles, are modified away from the 
 batrachian type, while those of the mammal are directly reducible to it. 
 Finally, the fact that in all Reptilia and Aves it is a right aortic arch, 
 which is the main conduit of arterial blood leaving the heart, while in 
 the Mammalia it is the left which performs this office, is a great stum- 
 bling-block in the way of the derivation of the jNIammalia from any of 
 the Reptilia or Aves. But if we suppose the earliest forms of both 
 Reptilia and Mammalia to have had a common batrachian origin, 
 then there is no difficulty in the supposition that from the first it was 
 the left aortic arch in the one series, and the right aortic arch in the 
 other, which became the predominant feeder of the arterial system." 
 
 If we had only the recent forms to consider, the argument advanced 
 by this learned anatomist would be specially potent ; but when we study 
 carefully the osteology of the Reptilia of the Permian Period, many of 
 the arguments here advanced are invalidated. The structure of the 
 pectoral and pelvic arches of the theromorph Reptilia, as has been ascer- 
 tained by Cope, resembles that of the monotremes far more than does 
 that of any known batrachian. The carpus and tarsus of these forms 
 are almost identical with those of the monotremes, while comparatively 
 
392 DENTAL ANATOMY. 
 
 little importance can be attached to the dicondylian character of the 
 skull, from the fact that there is in certain members of this group a 
 double articular siu-face on the occipital bone for the atlas vertebra. 
 The only ostcological character left in which the Batrachia resemble 
 the JMammalia most is that of the quadrate articulation ; which resem- 
 blance is somewhat counterbalanced by the approaches to the distincti\'e 
 peculiarities of the mammalian dentition found only in the Theiomovpha. 
 The condition of the arterial system must remain inferential for this 
 group, since it became extinct, so far as we now know, at the close 
 of the Permian Period. Ufion the whole, I am disposed to think that 
 there are Cjuite as many reasons to regard the theroniorph Reptilia 
 as the ancestors of the mammal as there are to regard in the same 
 light any of the Batrachia so far discovered. 
 
 Accepting the " placoid scale " or the " dermal denticle " as the struc- 
 ture from which all teeth were primarily derived, A\e have, as charac- 
 ters of a primitive dentition, the following : (1) the conical form ; (2) 
 increased number ; (3) frequent and almost endless succession. These 
 conditions we have tulfilled in many of the sharks. The next step in 
 specialization c(_)nsists in the fusion of the basal osseous plates of the 
 " dermal denticles " to form the maxillary and dentary bones, to which 
 the teeth become attached by ankylosis. This, we have already seen, 
 obtains in a majority of the fishes, and is associated largely with the 
 simple conical form. In the Batrachia the conical form, this mode of 
 attachment, as well as the succession, are closely adhered to, but the 
 individual teeth are reduced in number. In certain of the Reptilia — e. g. 
 Theromorphd — another advance is made in the implantation of the teeth 
 in distinct sockets, with a disposition to firm more than one root or 
 fang. There are still, however, many successive sets of teeth developed. 
 Lastly, in the Mammalia the teeth are generally greatly reduced in 
 number; they are always implanted by one or more roots in a distinct 
 socket, and there are never more than two sets developed, the second 
 of which is only partially complete ; they are also, as a general rule, 
 of a complex nature and show a ^vide departure from the primitive 
 cone. 
 
 In searching, therefore, for a primitive or generalized mammalian den- 
 tition, the most imjDortant point to be taken into consideration is the 
 following : numerous single-rooted teeth, confined to the maxillary and 
 mandibular bones, implanted in distinct sockets, with a complete devel- 
 opment of one or more successive sets. It is possible, even probable, 
 that tliis stage in tooth-de\'elopment was reached in the ancestors of the 
 Mammalia before they assumed their distinctive characteristics as such ; 
 but the nearer any approach is made to this condition on the part of the 
 mammal, in that proportion it may be regarded as primitive in its den- 
 tal organization. 
 
 Having already spoken of the probable origin of the Mammalia, it 
 now remains to give a brief synopsis of their classification before pro- 
 ceeding to a detailed description of their teeth. The arrangement here 
 ado]ited is, witli some modification, the one which has been proposed by 
 Prof E. D. Cope, and is based uptm a study of both fossil and recent 
 forms : 
 
TEETH OF THE VEBTEBBATA. 
 
 393 
 
 
 > 
 
 W 
 3 
 
 in 
 
 o 
 
 a 
 
 g 
 
 1-9 
 f 
 O 
 
 a 
 t> 
 
 rji 
 CD 
 
 
 3 B 
 
 re 3. 
 
 s- 
 
 §■ a 2. 
 
 o 
 3 
 
 3. 
 
 
 w 
 
 a 
 a 
 
 Ct 
 
 > 
 H 
 
 >— ( 
 O 
 
 o 
 
 l> 
 
 5;> 
 
 
 o 
 
 
 
 
 
 
 
 s 
 
 o 
 
 
 ^ 
 
 a- ^ 
 
 -!i3- 
 
 2^ 
 
 3 Cfi' 
 
 3 2. 
 ^ re 
 
 a- 2; 
 
 re ^ 
 5J crq 
 
 — CO 
 
 re 
 re 
 
 <S " o c 
 13.3 3 
 
 TJ 3'E° 
 
 ?a ^ *^ 3^ 
 ^ - — ■ p 
 
 re 
 
 c fD w 21. 
 
 fD f^ ^ 
 
 IT*- r^ 
 
394 DENTAL ANATOMY. 
 
 It will be seen, from the foregoing table, that the Mammalia 
 are divisible into two primary groups, Avhich hold the rank of sub- 
 classes. The first of these, Prototheria,^ has but two living repre- 
 sentati\'es, both of which are confined to the continent of Australia. 
 These are the Erhkhia, or spiny ant-eater, and the duck-billed platypus. 
 The principal characters by which they are separated from all other 
 Mammalia may be conveniently contrasted with those of the second 
 sub-class, Eutheria, as follows : in the former there are (1) "large and 
 distinct coracoid bones, which articulate with the sternum. (2) The 
 ureters and the genital ducts open into a cloaca into which the urinary 
 bladder has a separate opening. (3) The penis is traversed by a ure- 
 thral canal which opens into the cloaca posteriorly, and is not continuous 
 with the cystic urethra. (4) There is no vagina. (5) The mammary 
 glands have no teats." In the Euther'xt, on the other hand, (1) " the 
 coracoid bones are mere processes on the scaj)ula in the adult, and do 
 not articulate with the sternum. (2) The ureters open into the bladder, 
 the genital ducts into a urethra or vagina. (3) The cystic urethra is 
 continuous with the urethral canal of the penis. (4) There is a single 
 or double vagina. (5) The mammary glands have teats " (Huxley). 
 
 In their anatomical structure the Prototheria resemble the I'cptiles and 
 birds more than does any other mammal. This is particularly conspic- 
 uous in the pectoral arch and the reproductive system. On this account, 
 De Blainville applied tlie name Ornitlwdelphia (bird womb) to them, 
 by which they are sometimes known. Strange as it may seem, no fossil 
 remains of great antiquit}- of this most primitive group of all Mammalia 
 are with certainty known to exist, but it may yet be found that the 
 earliest mammalian representatives, which date as far back as the 
 Triassic Period, and which are known from teeth and jaw bones 
 only, really belong to the Prototheria rather than to the IHdelpMa or 
 pouched series of the Eutheria, as is frequently maintained. Both 
 the living forms are devoid of true teeth. 
 
 The second sub-class, Eutheria, has two principal divisions : DidelpMa 
 (double womb), including those animals popularly known as the 
 " pouched quadrupeds," of Avhich the opossum, kangaroo, wombat, 
 etc. are familiar examples ; and the Monodelphia (single womb), 
 which embraces all the remaining mammals. The name of the first 
 subdivision, JUdelj^hia, was applied by its author, De Blainville, with 
 reference to the peculiar habit which tliese animals possess of sheltering 
 their helpless young in an abdominal integumentary fold. This is corre- 
 lated with the only important character in which they differ from the 
 monodelph division — viz. the complete absence of an allantoic placenta 
 or any uterine connection bet\\een mother and foetus. In consequence 
 of this peculiarity of gestation the }'oung are born in an exceedingly 
 helpless and imperfect condition, and are nourished for a considerable 
 period in the marsupiuiii or pouch of the mother. This character is 
 
 ' The classification of the Rfamraalia proposed recently by Prof. Huxley includes 
 three principal subdivisions — viz. : Prototheria, Milniherin, and Fiilheria. The terms 
 Prntnlheria and Eiitlieria were employed by Prof. Gill a number of years previously to 
 designate the two principal groups of this class, and appear to have been appropriated 
 by Huxley without credit. 
 
TEETH OF THE VERTEBBATA. 395 
 
 considered of sufficient value by some to give the Didelphia a rank equal 
 to that of the Prototheria, and they consequently make three primary 
 divisions of the class — Oniithodelphla , Didelphia, &nd Monodelphia, a,iiev 
 De Blainville. If this ^vere associated with any other characters of 
 structural importance it would be quite sufficient, but since it is not, 
 and in view of its unreliability and inconstancy in the lower Vertebrata, 
 I am not disposed to regard it as equal in value to the strong structural 
 characters by which the Prototheria are defined. 
 
 The subdivision of the Monodelphia is not an easy matter, if indeed 
 any important divisions further than the separation of the mutilate 
 series can be made. It is convenient, however, to adopt the classifica- 
 tion of Lamarck, and divide them into three series, as follows : the 
 mutilate series, the ungulate series, and the unguieulate series. The 
 first of these includes the Cetacea, or whales, and the Sirenia, or sea- 
 cows. The only character by means of which they are associated 
 is the absence of hind limbs and the loss of the articular processes of 
 the bones of the manus. The Cetacea form a perfectly natural and 
 homogeneous group, and are entitled to a wide separation from all 
 other Mammalia. We at present know very little concerning their 
 development or ancestry, further than that their Eocene representative, 
 Zeuglodon, resembled the ordinary monodelphous type more than does 
 any other member of the order. They are undoubtedly a very old and 
 distinct group, and it would not be at all surprising if they are iij.tiraately 
 found to have descended directly and independently from the Prototheria. 
 
 The Sirenia, or sea-cows, on the other hand, ajipear to be simply 
 modified ungulates that have gradually assumed their present structure 
 in accordance with their aquatic environment. The Miocene genus 
 (Halitherium) of this order had distinct hind limbs, and in many ways 
 resembled the primitive hoofed Mammalia. For this reason it is 
 probably best to associate them with the ungulate rather than with 
 the mutilate series, since they differ in almost every essential feature 
 from the Cetacea, except in the loss of the posterior members. 
 
 The separation of the two remaining series, ungulate and unguieu- 
 late, depends entirely upon the distinctions to be drawn between " hoof" 
 and " claw." If we contrast, for example, two such structures as the 
 claw of the lion and the hoof of the horse, the distinctions are perfectly 
 obvious, and we will experience no difficulty in recognizing the ditfer- 
 ences ; but if we carefully trace the respective lines of ancestry of these 
 two forms backward to the Eocene Period, we will find them converging 
 to such an extent as to involve the hoof-and-claw question in almost 
 hopeless confusion. 
 
 There are, however, two principal lines or stems which have terminated 
 in the distinctly hoof-bearing mammals on the one hand and the claw- 
 and nail-bearing on the other. The exact point at which these two lines 
 converge has not as yet been satisfactorily determined, but it is undoubt- 
 edly true that they approached one another to a remarkable extent in 
 the early Eocene. The ancestry of the entire ungulate series is indi- 
 cated by the Taxeopoda of Cope, to whose persistent efforts and schol- 
 arly researches we are alone indebted for their discovery and description. 
 
 The primitive or central stem of this order is the Condylarthra, from 
 
396 DENTAL ANATOMY. 
 
 which we pass by easy stages through the extinct genus Menisootherium 
 to the little hyrax or " coney,'' ^^'hose classification has long remained 
 a puzzle to zoologists. From this group the extinct amblypods and 
 elephantoid animals likewise came, while the Perissodadyla and the 
 Artiodactyla are traceable directly to it. 
 
 The unguiculate series also has a generalized order, from which all 
 the others radiate in diiferent directions. This order has been called the 
 Bunotheria by Cope, and exhibits a central axis in the sub-order Insed- 
 ivorn, the representatives of which are among the oldest of monodelpli 
 mammals. From the Insectivora we derive the CS-eodonta, a group of 
 extinct insectivoro-carnivorous animals which terminates in the Car- 
 nirora. In another line come the lemuroids, monkeys, and man, while 
 in still another are the Cheiroptera or bats, ^vhich are simply insectivores 
 modified for flight. 
 
 One other order, the Edentata, or sloths, armadillos, ant-eaters, etc., 
 remains to be accounted for. Some authors believe them to be affiliated 
 with the unguiculate series, and to have sprung from the central insect- 
 ivorous group. Palaeontology has so far given us very few if any hints 
 concerning the origin of this order, and it is probable that it will not 
 be until the Eocene and Miocene Tertiaries of South America are more 
 fully explored that any important information will be had upon this 
 subject. At present I consider the evidence too meagre to hazard an 
 opinion. , 
 
 Divisions of the Majimalian Dentition. — Many years ago 
 Prof Owen called attention to the fact that in many of the Eidherw 
 there are two sets of teeth developed during the life of the individual — 
 a deciduous or milk set and a permanent set — while in others but a single 
 set appears. The former of these two conditions he designated by the 
 term di]ihyodont, and to the latter he gave the name monophyodont den- 
 tition. It likewise so happens that generally, in those that have two 
 sets (diphyodonts), the teeth in the various parts of the mouth are dif- 
 ferent in foi'm and complexity, while in those that have but a single set 
 (monophyodonts) the teeth are alike throughout. The diphyodont den- 
 tition is therefore, as a general rule, heterodont, that is, there are many 
 kinds of teeth, and the monophyodont dentition is homodont, or all 
 the teeth are alike. It was therefore originally supposed by Owen that 
 diphyodont and heterodont and monophyodont ancl homodont were cor- 
 relative and interchangeable terms, but it has since been discovered that 
 there are many exceptions to this rule. 
 
 It must be borne in mind that the terms diphyodont and monophyo- 
 dont are simply conveniences by Avliich we are enabled to express briefly 
 the conditions of replacement, and are not in any way to be looked upon 
 as definitiA-e of a natural group. The degree to which the second den- 
 tition is developed in the various sections of the Mammalia is subject to 
 extreme variation, and it is not always an easy matter, if not frequenth- 
 an utter impossibility, to determine whether certain teeth belong to the 
 deciduous or permanent set, or in the monophyodonts to say whether 
 it is the permanent or deciduous set which has been lost. There are, 
 however, as will appear later, several important series in \Yhich the 
 replacement and position are sufficiently constant to enable us to divide 
 
TEETH OF THE VERTEBRATA. 
 
 397 
 
 the teeth into several categories, the convenience of which, to say the 
 least, if not the real importance, is undeniable. The question of the 
 nature and relationship of the milk dentition to the permanent one 
 will be discussed after the teeth of the several groups have been 
 considered. 
 
 The Teeth and their Accessory Or(;ans in the Dog. — I have 
 thought best to next present a detailed description of the adult structure 
 of an average diphyodont dentition, together with the accessory organs, 
 in order that the student may become familiar with the technicalities 
 before proceeding to consider the teeth of the various sections of the 
 Mammalia. The dog has been selected as an example of this kind, 
 partially on account of the generalized condition of the teeth, but 
 principally on account of the readiness with which the student will 
 be enabled to provide himself with the necessary material. 
 
 The teeth of the dog (Figs. 195 and 196) are forty-two in number, 
 of which twenty belong to the upper and twenty-two to the lower 
 jaw. The most anterior teeth of the upper series are relatively small, 
 and are implanted in the free edge of the premaxillary bones in such a 
 manner as to describe the arc of a circle. These are known as the 
 incisors {ie, Figs. 195, 196). Behind these, after a slight interval, are 
 
 Fig. 195. 
 
 Vertical View of the U]pper J^w of a Dog {Canis familiaris) : ic, incisors ; c, canine ; pm, premolars ; 
 m, molars ; s, sectorial ; pmx, premaxiliary bone ; mx, maxillary bone ; pi, palatine ; apf, anterior 
 palatine foramen ; ppf, posterior palatine foramen. The position of the third premolar is slightly 
 abnormal. 
 
 a pair of strong, laterally compressed curved teeth, the canines, which are 
 lodged deeply in the substance of the maxillary bone, immediately behind 
 the maxillo-premaxillary suture. Behind the canines, again, are six 
 teeth on each side, which progressively increase in size and complexity 
 as we proceed backward until the penultimate tooth is reached, the last 
 one being somewhat smaller. These are termed molars and premolars. 
 The tooth-line of each moiety of the upper jaw presents three curves, 
 the most anterior of which is formed by the three incisors and canine, 
 with a strong convexity outward ; the line of the next four describes a 
 gentle curve whose convex surface is inward ; while that of the last two 
 curves boldly inward toward the median line. 
 
 The number of teeth in the lower jaw is one in excess of that of the 
 upper, which is caused by the addition of a small single-rooted tooth at 
 
398 
 
 DENTAL ANATOMY. 
 
 the posterior end, of the series. They describe the same curves, so as to 
 oppose those of the upper series. The incisors of the upper jaw, as lias 
 ah'eady been stated, are lodged in the pre- or intermaxillary bones, 
 which limit the anterior part of the oral cavity above. The definition, 
 therefore, of an incisor tooth of this series iw one ivhicli has a pre- or inter- 
 iiuixillary implantation irrespedirc of its size or foriii. The incisor teeth 
 of the lower jaw are the eorresponding ones icJiich are brought into opposi- 
 tion with, those of the upjperjaw trhen the mouth is dosed. The teeth thus 
 defi^ned are three in number upon each side above and below in this animal, 
 and are implanted by single slightly recurved fangs in distinct sockets or 
 alveoli. In the upper series the median pair is the smallest, the outer 
 ones gradually increasing in size. The base of the cro^^•lls of the four 
 middle teeth is somewhat trihedral in form, with the apex flattened from 
 before backward and produced into three cusps, of Avhich the central 
 one is the largest. The entire apex of the cr<_iwn is slightly recurved. 
 Upon its inner aspect the cro«)i presents a basal ledge or cingulum, 
 which sends out a low ridge to each of the lateral cusps. The lateral 
 incisors are the largest and are somewhat caniniform. Like the median 
 ones, their crowns have a strong basal cingulum posteriorly, but the 
 lateral cusps are absent ; the apex terminates in a strong hooked point. 
 
 The incisors of the lower jaw are similar to those of the upper, with 
 the exception of the median pair, which is much the smallest and occu- 
 pies a more antei'ior position than the others. The internal lateral cusps 
 of these teeth are very faintly indicated, if indeed the}' can be at all 
 made out, while the external lateral cusp is present and situated high 
 up in the two median pairs. In the lateral ones it has a position nearer 
 the base of the crown, and is separated from the median cusp by a deep 
 fissure. 
 
 Between the lateral incisors and canines of the upper series there is a 
 space or diastema about equal to the width of the lateral incisor. This 
 space serves to receive the lower canine when the mouth is closed. At 
 the back part of it upon the outside may be seen the suture by which 
 
 Fig. 196. 
 
 Vertical A'ieu- of the Lower Jaw of z Dog {C fitmUiaria) : ir, incisors; c, canine; pni, premolars; 
 
 m, molars; .^, -sectorial. 
 
 the premaxillary bone joins the maxillary in the dentigerous boi'der 
 of the jaw. Just behind this suture the superior canine is lodged. 
 
 The definition, then, of a superior canine tooth is one vliidi is situated 
 in the maxillary bone immediately behind the maxillo-prcmaxillarg suture, 
 proi'ided it be not too far back, tohatever may be its form, size, or func- 
 
TEETH OF THE VERTEBRATA. 399 
 
 tion, while the eanine of the lower jaw is the tooth which elosesjust in front 
 of it. 
 
 The canines of the dog are large, recurved, pointed teeth, projecting 
 far above the level of the others, with slightly trenchant anterior and 
 posterior edges. They are almost equal in size and very similar in 
 shape. A very useful means by which tiiey can be distinguished from eacli 
 other, if at all worn and isolated from the rest of the teeth, is to note 
 the point at which the worn surface exhibits itself It must be remem- 
 bered that the lower canine closes in front of the upper, in consequence 
 of which the j^ostcrior face. of the lower impinges against and abrades 
 the anterior face of the upper; the anterior face of the lower canine also 
 comes in contact with the lateral incisor, and an abrasion takes place at 
 this point ; but the posterior face of the upper canine is seldom worn 
 except by long-continued use, so that ordinarily these points of wear 
 serve as a useful guide in distinguishing between them. There is a 
 slight difference in form, which can be ascertained only by close and 
 careful comparison. 
 
 Behind the canines are four teeth which have been designated praao- 
 lars. The reason for this distinction is founded upon the circumstance 
 that these are the teeth situated behind the canine which vertically suc- 
 ceed the corresponding ones of the deciduous or milk set. The defini- 
 tion, therefore, of a premolar tooth is one ivhich, being situated behind 
 the canine, displaces in a vertical direction a deciduous or milk tooth; all 
 others behind these are true molars. This is the definition which was 
 originally proposed by Owen, to whom we are greatly indebted for this 
 nomenclature : it would apjjcar to be entirely satisfactory and sufficient, 
 were it not for the fact that the first tooth counting from before back- 
 ward, which is generally enumerated in the premolar series, does not 
 have any deciduous predecessor. If we adhere strictly to this definition, 
 it cannot be justly considered a premolar, but common usage has so 
 long given it a place in this category that it appears advisable to 
 still call it such. It should be remembered, however, that this is by 
 no means an isolated case, but that other animals exhibit similar 
 peculiarities. 
 
 The first premolar,, so called, of the superior series is the smallest of 
 the four, and is implanted rather obliquely in the maxillary bone ; its 
 single fang is slightly compressed laterally, and joins the crown at a 
 moderately well-defined neck. The crown has an elongated oval form, 
 terminated by a prominent obtuse cu.sp and surrounded by a well- 
 marked ledge or cingulum, which is most conspicuous upon its inner 
 face. From the summit of the main cusp two well-defined ridges 
 descend to the cingulum, one on the posterior and the other upon the 
 anterior border, giving to the tooth a slightly trenchant appearance. 
 The hindmost of these two ridges divides the posterior half of the 
 crown into two equal parts, and terminates with a very slight enlarge- 
 ment in the cingulum, while the anterior one has a more internal direc- 
 tion, and terminates in a distinct tubercle which occupies a position at 
 the base of the antero-internal portion of the crown. The two ridges 
 and the cingulum below enclose a shallow triangular depression inter- 
 nally, the outer face being convex. 
 
400 BEJSITAL ANATOMY. 
 
 The second and third premolars are considerably larger than the first, 
 and are implanted by two roots, of which the posterior is the larger. 
 These U\o teeth resemble one another very closely, the only appreciable 
 difference being their slight disparity in size. Their crocus, like that of 
 the lirst, are of greater longitudinal than transverse extent, and are pro- 
 duced into a prominent cusp situated a little anterior to the centre. The 
 posterior ridge is interrupted shortly before it joins the cingulum by a 
 (jeejj transverse notch which gives rise to a distinct viit^p, the posterior 
 basal tubercle, situated over the hinder root. A faint indication of a 
 second cusp is seen just behind this as an elevation of the cingulum. 
 The antero-internal tubercle is present, and occnjiies relatively the same 
 position as it does in the first premolar. The cingulum is more jiromi- 
 nent on the inner than on the outer side of the crown, and with the two 
 ridges encloses a triangular space. 
 
 The fourth premolar is by far the largest and strongest tooth of the 
 premolar series. It is commonly known as the " flesh tooth," or sujye- 
 rior sectorial, for reasons presently to be given. It is implanted by 
 three roots, two external and one internal. The crown is composed of 
 two principal lobes supported by the tA\"o external roots, and a small 
 antero-internal one suj)ported by the internal fang The two prin- 
 cipal lobes have an antero-posterior jjosition, and are separated from 
 each other by a deep, narrow fissure. Of these, the anterior is the 
 larger and higher of the two ; when viewed externally it resembles a 
 cone with the anterior contour produced. Internally it is fl^attened 
 somewhat, so as to correspond with the flattened inner surface of the 
 posterior lobe. Posteriorly it is jiroduccd into a strong blade-like ridge, 
 which is terminated by the vertic'al fissure, while its anterior surface is 
 marked by a moderate vei'tical ridge. The posterior lobe is essentially 
 ehistl-shaped in form, with the bevelled edge external ; its apex forms 
 a blade-like ci'est \\hich extends the entire length of the lobe. The 
 internal lobe is small, and occujiit's a jiosition at the antero-internal 
 angle of the crown, being conncctt'd with a faintly-marked cingulum 
 which siuTounds the base of the crown. ^Allen we attempt to homol- 
 ogize the component lolies of this tooth with those of the premolars in 
 advance of it, it is not difficult to see that the anterior lobe is the prin- 
 cipal cone, that the posterior mw is merely an exaggerated posterior 
 basal tubercle, while the internal lobe is strictly homologous with the 
 structure which has a similar position in the others. The three anterior 
 premolars are not in as close contact as the teeth in the back part 
 of the jaw, but are separated from each other by slight intervals, which 
 are most conspicuous between the first and second. 
 
 The premolars of the louver jaw are similar in form to those of the 
 upper, with the imjjortant excejition of the fourth or last, wherein 
 tliere is to be found a wide difference both in size and structure. 
 The first of the lower series is smaller than the corresponding tooth 
 aljo\-e, and lias a simpler, more conical croAvn. It is separated bv a 
 considerable diastema from the canine in front of it, but is almost in 
 (■(intact with the second behind. The second and third resemble those ' 
 which are in a like position in the upper jaw, while the fourth is also 
 similar to the corresponding tooth above, with the exception of a 
 
TEETH OF THE VEBTEBRATA. 401 
 
 slightly increased size and the possession of a well-deiined second 
 posterior basal lobe. It slightly overlaps the great first true molar 
 behind it. 
 
 The true molars of the superior series are two in number upon each 
 side, and are placed directly behind the premolars. The definition of a 
 true molar tooth is one which, being situated behind the premolars, does 
 not displace a deciduous or milk predecessor. The two molars above 
 are three-rooted, with broad tuberculated crowns imperfectly quadran- 
 gular in outline. The first, which is more than twice the size of the 
 second, has two strong obtuse conical tubercles on the external portion 
 of the crown, situated directly over the anterior and posterior external 
 roots ; they are subequal and separated from each other by a transverse 
 notch. Internal to these there is a broad ledge, well rounded off inter- 
 nally, bearing three cusps. The one most internal is lunate in form, and 
 is closely connected with the cingulum, which surrounds the base of the 
 tooth. The cusp situated near the antero-internal angle is the largest, 
 and has a subtribedral form. A distinct ridge passes outward and for- 
 ward from it to join the cingulum. Posterior to this last-mentioned 
 cusp, and separated from it by a wide open notch, is the third tubercle, 
 less distinctly marked than either of the others. An analysis of the 
 various cusps of which the crown is composed leaves little room to 
 doubt that the two external cusps are strictly homologous with the two 
 external ones of the sectorial in advance of it — that the internal ledge 
 which bears the three tubercles represents the greatly enlarged internal 
 lobe of the sectorial, which has been removed to a more posterior 
 position, and has acquired an important addition from the cingulum. 
 That part of it which is exactly homologous with the internal lobe 
 is the principal cusp at the antero-internal angle, which in some car- 
 nivorous animals is continued outward and backward as a prominent 
 ridge, and does not develop the third tubercle. If the lunate cingular 
 cusp be subtracted, the crown will be seen to resemble that of the 
 sectorial in its general features. 
 
 The second true molar is like the first, except that the internal ledge 
 exhibits, instead of three tubercles, two crescentiform ridges. 
 
 The first true molar of the lower jaw is the largest tooth in the entire 
 dentition of the dog, and is the sectorial of the inferior series. It is 
 implanted by two powerful roots at a point about midway between the 
 anterior extremity and the condyle of the lower jaw, and occupies a 
 position near the canthus or angle of the mouth. Its crown may be 
 described as composed of two anterior blade-like cusps, a small inter- 
 nal tubercle, and a low tubercular Ireel. Of the two anterior cusps, the 
 posterior is the larger, and rises gradually above the level of the one 
 anterior to it; both are convex internally, but somewhat flattened exter- 
 nally to correspond with the internal flattened surface of the two blades 
 of the superior sectorial. They are separated from each other by a deep, 
 narrow fissure. The heel is low, and occupied by two cusps disposed 
 transversely, of which the outer one is the larger. A faint ridge con- 
 nects them, enclosing a shallow basin in front ; on this account the heel 
 is said to be basin-shaped. The internal tubercle is small, and is placed 
 at the inner posterior part of the median lobe. In many carnivores it 
 
 Vol. I.— 26 
 
402 
 
 DENTAL ANATOMY. 
 
 completely disappears, as does also the heel, as we shall presently see. 
 The fourth superior premolar and the first inferior true molar are called 
 sectorial, on account of their seissor-blade structure and the manner in 
 which they oppose each other. If the macerated skull of a dog be care- 
 fully examined, it will be seen that the incisors of each series oppose 
 each other almost exactly, while the lower canines close in front of the 
 upper. As a conse(|ueiice of this, the first premolar below closes in 
 advance of tiie first premolar above ; the second belo-w in the interval 
 between the first and second above, etc., but always upon the inside, on 
 account of the unequal width of the two jaws. Xow, the inferior sec- 
 torial bites against the superior in such a manner that its blades exactly 
 oppose those of the tooth above after the manner of a pair of shears, so 
 that when the mouth is closed the inferior sectorial is completely hidden 
 from view ; the heel ojoposes the first true molar a))ove. Those who 
 have ever studied the habits of dogs or w(jlves must have noticed that 
 'when they wish to divide a tough animal membrane or ligament they 
 pass it back to the canthus of the mouth on one side and make several 
 short (juick strokes of the jaw ; this is the shearing movement of the 
 sectorials. 
 
 The remaining two true molars are much smaller, the last being one 
 of the smallest of all the teeth, and is implanted by a single root. It is 
 said to be permanently absent in some races of the domestic dog, espe- 
 cially the " pugs " and " Japanese sleeve dogs." The second molar is 
 two-rooted, with a tuberculate crown of a more or less quadrate form. 
 Two transverse cusps occupy the anterior part, "s\-hile a third is placed 
 at the postero-external angle of the crown on the edge of a broad flat 
 heel. A basal cingulum is also present. The last tooth has an obtusely 
 
 Fig. 197. 
 
 Side View of the Skull of a Dog (C.famitiaris). 
 
 conical crown. The homologies of the cusps of the inferior true molars 
 are not evident in the dog, but when we come to examine allied forms 
 it will be found that the two blades of the sectorial represent the primi- 
 tive cone, and the anterior basal lobe of the ordinary premolar greatly 
 enlarged and specialized, while the heel represents the two posterior 
 
TEETH OF THE VEBTEBRATA. 403 
 
 basal tubercles arranged transversely ; the internal tubercle is an extra 
 outgrowth from the cingulum. 
 
 In the case of many extinct animals the succession, and consequently 
 the discrimination, of the molars and premolars would be attended with 
 considerable difficulty were it not for the fact that in a majority of the 
 Mammalia the first true molar is the first of the permanent yet of the 
 molar and premolar series which comes into place. By the time the 
 last or fourth premolar is cut, which is usually one of the last, the first 
 true molar immediately behind it is considerably worn down by use, so 
 that this disparity of wear will of itself frequently serve to locate the 
 exact limits of each series. It is a rule which is often employed by 
 palseontologists to determine the dental formula of an animal the suc- 
 cession of whose teeth is unknown. When the anatomist wishes to 
 indicate briefly the number of the various teeth of any particular ani- 
 mal, he employs what is called a dental formula. By this method the 
 permanent dentition of the dog would be expressed as follows : I. |, C. 
 \-, Pm. 1^, M. -| ; which means that there are three incisors upon each 
 side above and below, that there is one canine upon each side above and 
 below, that there are four premolars, and that there are two true molars 
 above and three below. This manner of abbreviation is convenient and 
 easily understood, and saves both time and space in descriptions. 
 
 The division of the teeth into incisors, canines, premolars, and molars, 
 although open to some objection, is nevertheless useful, since it serves to 
 locate, in the case of addition or subtraction of a tooth to or from the nor- 
 mal diphyodont number, the exact position in which the change has 
 taken place. In the marmoset monkeys of South America, for exam- 
 ple, the total number of teeth is thirty-two, the same as in man. An 
 inspection of their formula, however, which is I. f , C. \, Pm. -I, M. -^ 
 = 32, will show that there is an important difference between the num- 
 ber of molars and premolars, the formula in man being 1. 1-, C. \, Pm. 
 1^, M. f. In the former it is a molar vihich is lost ; in the latter it is a 
 premolar. Another example of this kind is seen in the upper and lower 
 teeth of the otter, in which they are equal in total number, but unequal 
 as far as the respective kinds are concerned. The dental formula in this 
 animal is I. |, C. |, Pm. f , M. i = 36. 
 
 The Accessory Organs. — This subject properly embraces a considera- 
 tion of the bones by which the teeth are supported, the muscles con- 
 cerned in their movement, the blood-vessels by which they are supplied 
 with nutriment, and the nerves distributed to them. The bones in 
 which the upper teeth are implanted are the maxUhe and prcmaxiUce, 
 which are usually enumerated as bones of the face. The maxillary bone 
 (Fig. 195, mx) is by far the largest one belonging to this category, and 
 forms the greater part of each moiety of the upper jaw. It likewise 
 contributes to the formation of the cheek, orbit, and palate, and also 
 takes the principal share in forming the boundary of the nasal cham- 
 ber. The maxillary bones do not meet in the median line above, on 
 account of the interposition of the nasals and premaxillaries, but 
 below they send inward two thin horizontal plates which meet in 
 the middle of the roof of the mouth. 
 
 For descriptive purposes it is convenient to divide each bone into 
 
404 DENTAL ANATOMY. 
 
 three external surfaces, the facial, jDalatine, and orbital. The facial sur- 
 face, A\'hich is the largest of the three, is directed outward, and is irregu- 
 larly triangular in form, with the apex directed forward. The superior - 
 border of this surface is considerably curved, and joins the premaxillary 
 in front and the nasal behind. The posterior border is irregular, and is 
 in contact from within outward with the frontal, lachrymal, and malar 
 bones respectively, being excluded by these bones from the rim of the 
 orbit. The inferior border is known as the dental or alveolar border, 
 on account of its affording support to the canine, premolar, and molar 
 teeth of the upper jaw. It is in contact with the premaxillary in front, 
 and tei'minates behind in a free extremity beneath the orbital fossa. 
 
 The surface thus bounded is uneven, being interrupted by elevations 
 and depressi(_)ns. At the anterior angle the superior canine is implanted, 
 and the course of its powerful curved root is indicated by a well-marked 
 ridge or swelling of the external surface. Behind and above the pos- 
 terior termination of this is a broad, shallow depression, while behind 
 and below is another depression, the canine fossa, ending posteriorly in 
 a large foramen, the infraorbital foramen, situated above the interval 
 between the third and fourth premolars. Behind the infraorbital for- 
 amen a strong process is thrown up to meet the malar ; this is known 
 as the malar process of the maxillary. The posterior superior angle is 
 produced into a considerable rounded process, which passes as far back- 
 ward as the centre of the orbit to articulate with the frontal. This is 
 the nasal process of the maxillary, and is the hoinologue of a corre- 
 sponding process in the human skull bearing this name. 
 
 The posterior or orbital surface is relatively small, convex from before 
 backward, and concave from side to side. It is somewhat triangular in 
 shape, and forms the greater part of the floor of the orbit, being directed 
 upward and backward. It is bounded above and externally by the 
 malar, directly above by the lachrymal, and internally by the palatine 
 bones respectively, terminating in a free rounded border behind. The 
 internal portion of this last-mentioned border is separated from the 
 palatine by a notch, and forms a conspicuous eminence known as the 
 maxillary hiherosiiy. At the anterior extremity of this surface is seen 
 the posterior opening of the infraorbital canal, -which traverses the max- 
 illary bone and serves for the transmission of the second division of the 
 trigeminal or fifth nerve, as well as a part of the external carotid artery, 
 which terminates in this situation as the infraorbital. This surface is 
 perforated by small foramina for the entrance of the superior dental 
 ner\-es and arteries. 
 
 The inferior or palatal surface forms a considerable part of the bony 
 roof of the mouth as well as the floor of the nasal chamber. It is limited 
 in front by the joremaxillary bone, externally by the free alveolar border, 
 posteriorly in part by the palatine and in part liv a free edge, and inter- 
 nally by the suture with which it joins its tellow of the opposite side. It 
 is slightly concave from side to side, the alvecilar border being consider- 
 al)ly elevated. Posteriorly it sends back^N'ard a narrow strip which ter- 
 minates in a free edge behind ; anterior to this, at a point opposite to the 
 anterior part of the sectorial, it widens rapidly. From this point 
 to its anterior termination it gradually narrows again. Just internal 
 
TEETH OF THE VERTEBBATA. 405 
 
 to the sectorial is seen a deep depression, the sectorial fossa, which 
 serves to accommodate the blades of the inferior sectorial when tlie 
 mouth is closed. Internal to this, again, are usually two, sometimes 
 three, foramina, the posterior palatine foramina, which transmit the pos- 
 terior palatine vessels and nerve. From the largest, most anterior of 
 these a shallow groove is continued forward in which the palatine ar- 
 tery is lodged. This is the palatine groove. 
 
 The line of junction of the two palatal plates of the maxillaries is 
 marked by a longitudinal ridge, the sutural ridge, which gives support 
 to the vomer above. The maxilla articulates with the premaxilla and 
 nasal in front, with the frontal above, and with the lachrymal, malar, 
 and palatine behind. 
 
 The premaxillse are small bones placed in front of the maxillae, the 
 two together forming the anterior termination of the upper jaw. Each 
 consists of a thickened anterior portion meeting in the median line, 
 together with an ascending or vertical process and a horizontal process. 
 The thickened body forms the lower boundary of the anterior nares, 
 and by its free alveolar border lodges the incisor teeth. The ascending 
 or vertical process is a long, sharp spicule of bone which springs from 
 the outer side of the body and furnishes the external wall for the narial 
 opening. It is directed upward and backward, and insinuates itself 
 between the nasal above and the maxilla below. This is known as the 
 nasal process of the premaxilla. Upon either side of the median line 
 the horizontal or palatine processes pass backward to the maxillae, form- 
 ing the anterior portion of the bony palate. These processes are in con- 
 tact with each other in the middle line, but each is separated from its 
 body by a wide hiatus, which is converted into a foramen by the inter- 
 position of the palatal plate of the maxillary behind. These large oval 
 foramina are conspicuous features in the macerated skull, and are known 
 as the incisive or anterior palatine foramina. They transmit the ante- 
 rior palatine vessels and nerve. 
 
 The next and last bony structure to be noticed in connection with the 
 teeth is the mandible or lower jaw. This part of the skeleton in human 
 anatomy is known as the inferior maxilla, and consists, in the adult state 
 at least, of a single bone (the two halves co-ossified), as is also the case 
 in the monkeys and several other mammals. In the majority of them, 
 however, it is made up of two more or less persistent pieces, which may 
 unite in extreme old age to form a single bone. The mandible of the 
 dog consists of two symmetrical elongated halves, the rami, diverging 
 behind and coming in contact in front in the median line by two rough- 
 ened surfaces, the symphysis. They are bound together by the inter- 
 position of fibro-cartilage at this point, and are movably articulated to the 
 skull behind by two transversely elongated processes, the mandibular con- 
 dyles, placed near the middle of the posterior border. Each ramus is 
 laterally flattened, with the inferior border considerably curved in an 
 antero-posterior direction. In front this border slopes gradually upward 
 to meet the alveolar or dentary border, while behind it is terminated by 
 a prominent, slightly-inflected process, the angle. The dentary border is 
 nearly straight, and is prevented from reaching the posterior border by 
 the intervention of a broad flat recurved plate of bone, the coronoid pro- 
 
406 DENTAL ANATOMY. 
 
 cess, which rises high above the level of the surrounding parts. The 
 posterior border is interrupted by two notches, between which is situ- 
 ated the condyle. Immediately in front of the condyle is a wide and 
 deep depression, the masseteric fossa, for the insertion of the powerful 
 masseter muscle. In front of and below the condyle, on the inner side, 
 is a conspicuous opening, the inferior dental canal, which gives passage 
 to the inferior dental artery and nerve. On the external surface, behind 
 and below the root of the canine, is another opening, the mental foramen, 
 through which a part of the nerve makes its exit to be distributed to the 
 lower lip. 
 
 The Jhiscles. — The principal muscles concerned in the movement of 
 the lower jaw are the temporal, masseter, external, and internal ptery- 
 goids, the digastric, genio-hyoid, and mylo-hyoid. 
 
 The Temporal is a broad, thick, fleshy muscle which covers the side 
 wall of the brain-case from the post-orbital process in front to the lamb- 
 doidal or occipital crest behind, reaching as high up as the sagittal crest 
 above, and completely filling up the temporal fossa, to which it gives its 
 name. Its fibres converge fan-wise to be inserted into the summit of 
 the coronoid process of the ramus of the mandible. Its principal action 
 is to elevate the lower jaw. By its leverage and great strength the ani- 
 mal is enabled to take a firm grip upon its prey. 
 
 The Masseter is a short, thick muscle arising from the under and a 
 part of the outer surface of the malar bone, as well as the posterior part 
 of the maxillary, and, passing downward and backward, is inserted into 
 the masseteric fossa of the ramus. Its action is similar to that of the 
 preceding muscle. 
 
 The Internal Pterygoid muscle consists of a strong bundle of muscu- 
 lar fibres which takes its origin from the pterygoid fossa in the base of 
 the skull, and passes downM'ard and outward to its insertion in the lower 
 part of the angular process. By its contraction the lower jaw is drawn 
 upward and inward, but owing to the manner in which the teeth inter- 
 lock no extensive lateral movement is possible. The most reasonable 
 view of the action of this muscle, as well as the succeeding one, is, 
 that by the contraction of those of one side the sectorial apparatus of the 
 side opposite is enabled to perform a more perfect shearing movement, 
 just as the blades of a pair of scissors must be pressed closely together 
 in order to make them cut. From the direction of its fibres it likewise 
 assists in elevating the jaw. 
 
 The External Pterygoid arises from the pterygoid plate of the sphe- 
 noid bone, and is inserted into the base of the condyle, and as far 
 forward as the inferior dental canal. Its action has already been 
 alluded to. 
 
 The Pigastrie is a large muscle which arises from the skull behind 
 the auditory bulla in a strong bony prominence, the paramastoid pro- 
 cess, and ]>asses forward to its attachment on the inferior margin of the 
 ranms in front of the angular process. Its action is to depress the jaw 
 and open the mouth. 
 
 The Genio- and Mylo-hyoid muscles are broad muscular sheets which 
 lie between the rami forming the floor of the mouth iu the recent state 
 being attached to the hyoid bones and the " fork " of the jaw. They 
 
TEETH OF THE VERTEBRATA. 407 
 
 assist in depressing the mandible, and consequently in opening the 
 mouth. 
 
 Vessels and Nerves. — The blood-vessels by which the teeth and the 
 muscles described above are supplied are derived from the external 
 carotid artery, which passes forward aloug the side of the neck, giving 
 oif branches to the various structures in this situation. This artery does 
 not terminate, as in man, in the temporal and internal maxillary arteries 
 — at least it is so generally considered by anatomists. Both the right 
 and left common carotids spring from the innominate, as in the Camlv- 
 ora generally. After giving off the thyro-laryngeal branch, remarkable 
 for its large size, to the thyroid gland and larynx, it passes forward in 
 front of the transverse process of the atlas vertebra, where it gives off 
 the occipital artery, which goes to the back of the head and the deep 
 muscles of the neck. Upon the base of the skull in the vicinity of the 
 carotid canal it bifurcates into two principal branches, the external and 
 internal carotids, the latter entering the skull through this canal to be 
 distributed to the brain, the latter continuing forward through the ali- 
 sphenoid canal, giving off in its course the laryngeal, lingual, facial, 
 posterior auricular, and superficial temporal branches. Near the con- 
 dyle of the lower jaw it describes a remarkable sigmoid curvature be- 
 tween this structure and the internal pterygoid muscle, thence passing 
 forward to the infraorbital canal, where it receives the name of the 
 infraorbital artery. Between the condyle and the infraorbital canal the 
 following principal branches are emitted by this arterial trunk : the 
 inferior dental artery, which enters the inferior dental canal and sup- 
 plies the teeth of the lower jaw ; the deep posterior temporal, which 
 furnishes a masseteric branch passing through the sigmoid notch, or that 
 between the condyle and coronoid process of the ramus, to enter the 
 masseter muscle ; several pterygoid arteries, which go to the pterygoid 
 muscles ; the ophthalmic artery, distributed to the eye ; the deep ante- 
 rior temporal ; the palatine, buccal, and alveolar arteries ; lastly, the 
 superior dental artery, which supplies the teeth of the upper jaw. 
 
 The nerves supplying the teeth and accessory organs are derived 
 principally from the trigeminal or fifth pair of cranial nerves. This 
 is essentially a mixed nerve in function, arising by two roots, a large 
 sensory and a small motor root. At a short distance from its origin the 
 sensory root swells out into ganglionic enlargement, the Gasserian gan- 
 glion, after which it divides into three branches — the ojMhalmic ov first 
 division, the superior maxillary or second division, and the inferior max- 
 illary or third division. 
 
 The first of these makes its exit from the cranial cavity through the 
 sphenoidal fissure, and supplies by its subdivisions the eyeball, mucous 
 membrane of the eyelids, the skin of the nose and forehead, dividing into 
 frontal, lachrymal, and nasal branches. 
 
 The second of these branches, the superior maxillary, issues from the 
 skull through the foramen rotundum, and supplies the side of the nose, 
 upper teeth, and the upper pai't of the mouth and pharynx. It crosses 
 from the foramen rotundum directly to the infraorbital canal, in the 
 vicinity of which it gives off the anterior and posterior dental nerves 
 which supply the teeth. 
 
408 DENTAL ANATOMY. 
 
 The third division, inferior maxillary, passes out of the skull through 
 the foramen ovale, just outside of which it is joined by the motor root. 
 It then divides into two branches, a small anterior one distributed to 
 the muscles of mastication, and a large posterior branch, which supplies 
 the ear, side of the head, lower lips, gums, teeth, salivary glands, and 
 inside of the mouth. The posterior branch divides into the aurieulo-tem- 
 poral, which passes backward to the temporal region ; the inferior dental, 
 which sujjplies the teeth of the lower jaw ; and the gustatoi-y, or the nerve 
 of taste, \vhich goes to the mucous membrane of the tongue. 
 
 The lips, tongue, and salivary glands should also be mentioned in 
 connection with the accessory organs, since they serve an important 
 purpose in preventing small particles of the food from escaping during 
 mastication, as ^^'ell as supply the requisite moistening fluid whereby 
 comminution is more readily accomplished and the food rendered more 
 digestible. 
 
 Teeth of the Edentata, or Bruta. 
 
 Although this group is by no means the most primitive of the Mam- 
 malia, as will be seen by reference to the table of classification, yet the cha- 
 ractei's which we have assigned to the ideal primitive mammalian dentition 
 are most nearly approached in certain members of this order. Whether 
 the comparatively simple form and absence of enamel in the adult tooth, 
 which is characteristic of all the animals of this order, pertain to a prim- 
 itive state, or whether this condition has been reached by a process of 
 retrogression or degradation, as many believe, we are not at present pre- 
 pared to say, in the absence of any knowledge of their palseontological 
 history beyond the latest Tertiary epoch. There is one character, hew- 
 ever, in which one at least is more decidedly primitive than any other 
 known Eutherian mammal, and that is the succession of all the teeth but 
 one (the last) by a second set. I refer to the nine-banded armadillo 
 {Tdtnsia peba). It is not certainly known whether this condition exists 
 in any other of the edentates or not, Avith the exception of the sloths, 
 wliich are truly mono]3hyodont. 
 
 The term Edentata is inappropriate, inasmuch as one would \x led to 
 suppose from the name that they have no teeth. The original term, 
 Bruta, was applied to this order by Linntens, ^^■hich he defined by the 
 absence of incisor teeth. It was afterward changed to Edentata hv 
 Cuvier — a name A-^hich has been extensively adopted l)v sulisequent 
 authors. It was formerly snjiposed that no incisor teeth are ever pres- 
 ent in this group, but the discovery of new fcjrms proved this to be 
 erroneous. The median incisors, however, are wanting in all cases so 
 far known. The definition of the order now most commonly given 
 is " absence of enamel on the teeth." This peculiarity appears at first 
 siglit striking and quite sufficient to se]iarate them from all other mono- 
 delphous mammals, but C. S. Tomes lias sliown' that the tooth-germs 
 of the nine-banded armadillo have distinct enamel organs, which are 
 subsequently aborted as the tooth comes to maturity. This discoA-ery is 
 important, since it indicates pretty clearly that the loss of enamel is a 
 
 ^ Philos. Trans., 1S76. 
 
TEETH OF THE VEBTEBRATA. 
 
 409 
 
 mark of degeneracy, and leads indirectly to the conclusion that the 
 armadillos at least are descended from ancestors with enamel-covered 
 teeth, who in all probability were the possessors of a completely devel- 
 oped second set. 
 
 The only assignable cause for this degenerate condition of the dental 
 organs is the peculiarity of their food-getting habits. Many of them 
 feed upon insects, which they capture by means of a long whip-like 
 tongue covered with the viscid secretion of the submaxillary glands, and 
 swallow whole. This manner of feeding would occasion little demand 
 for masticatory organs, which from disuse would gradually fall into a 
 rudimentary condition and eventually disappear. Those in which the 
 entomophagous habit is exhibited in its greatest perfection are edentu- 
 lous, and have small mouths with extremely long tongues. All the 
 Edentata in which this structure exists at all show a tendency toward 
 such a habit — even the arboreal sloths, which are said to be exclusively 
 vegetable feeders. 
 
 Flower has recently shown that the. sloths are intimately connected 
 with the ant-eaters and armadillos of South America through the extinct 
 megatheroids, and that all the American forms have probably descended 
 from a common ancestor, while the Old World forms are likewise closely 
 related and descended in another line. It is probably true that the arma- 
 dillos are most nearly related to the ancestral fonn, and that the sloths 
 represent an oflPshoot which was derived from them after they had lost 
 the enamel of the teeih in the manner indicated. 
 
 The teeth of the armadillos are, with one exception, relatively small 
 cylindrical bodies implanted in the dentigerous borders of the lower jaw, 
 maxillary, and sometimes premaxillary bones. They are entirely devoid 
 of enamel, and grow continuously throughout the life of the animal, in 
 consequence of which no roots are formed. 
 
 Fig. 198. 
 
 Side View of the Skxill of a Seven-banded Armadillo {Tatima hybrldusf) 
 
 In the seven-banded armadillo ( Tafotsia hybridus, Fig. 198^) there are 
 seven teeth above and eight below upon each side. So little is known 
 
 ' The specimen here figured is in the U. S. Army Medical Museum, and is labelled 
 Tatusia seplemeinctits. It exhibits the peculiarity of having eight teeth upon one side 
 and seven upon the other in the upper jaw. There is, however, a considerable space 
 between the first and second tooth of the right side, which -would indicate that a tooth 
 is missing. The number ascribed to this species by Owen is seven above and eight 
 below upon each side. Its exact identification is therefore difficult. 
 
410 DENTAL ANATOMY. 
 
 about their succession that it is impossible to say whether there are 
 molars and premolars represented or not. The teeth of the upper series 
 are lodged in the maxillary hones, and begin at a considerable distance 
 behind the maxillo-premaxillary suture. They progressively increase 
 in size up to the fifth or sixth tooth, the last being quite small. They 
 are not in contact with each other, but are separated by slight spaces 
 about ec^ual to the width of a tooth. The teeth of the lower jaw are 
 similar t(j those of the upper jaw in size and shape, with the exception 
 of the last, which is much larger than the corresponding tooth above. 
 The teeth of the inferior series close in the intervals between those of 
 the upper and conversel}-, causing the summits of the crowns to wear, 
 as Prof. Owen puts it, " into t-wo facets divided by a median transverse 
 ridge." The form of the working surface of the tooth is therefore 
 wedge-shaped. The first two teeth of the lo^ver jaw shut in front of 
 the first tooth above, and the last three teeth above behind the last one 
 of the lo\\'er series, leaving them with little or no opposition. Each 
 tooth continues its cylindriform shape to the bottom of the alveolus in 
 which it is implanted, having its base excavated into a large pulp- 
 cavity. It consists of dentine and cementum only. 
 
 In another species, the nine-banded armadillo, the number and form 
 of tiie teeth are the same. The teeth of this animal, as has already been 
 stated, have a successional set. According to the definition laid down 
 for premolar and molar teeth in the diphyodont Mammalia generally, 
 there would be one molar and six premolars in the dentition of this 
 animal. The rooted appearance of the deciduous teeth, according to 
 Tomes, is not due to the jjossession of true roots, but to the absorp- 
 tion set up l3y the approach of the successors. 
 
 The genus Prioflon of this group has as many as one hundred teeth, 
 the greatest number exhibited by any land mammal. They are rela- 
 tively small and simple in form, and are confined to the maxillary and 
 mandibular bones. Tliey vary in number from twenty-four to twenty- 
 six upon each side in the upper, and from twenty-tAvo to twenty-four 
 upon each side in the lower jaw. In the living genus Dasifput< there is 
 one tooth upon each side implanted in the premaxillary bone, ■which, 
 according to the definiti<in, becomes an incisor, \vliile in still another 
 extinct genus, Chhnnydotherium, almost equalling in size the rhinoceros, 
 there M'ere two incisors above and three which opposed them below. In 
 (ilyptodon the teeth are more complex in pattern, being laterallv com- 
 pressed and divided by twD vertical grooves upon each side, which are 
 opposite to each other. The resulting structure from this arrangement 
 is three ti-ansverse vertical plates connected in the centre bv an isthmus. 
 Tliere were teeth in the premaxillaries in this genus. 
 
 The mcgatheroids afford another example of moderate complexity in 
 the enamelless teeth of the Brufa. In the gigantic extinct Megcdherinin 
 there are five molars above and four below upon each side. They are 
 very deeply implanted in the substance of the jaw bones, and have 
 remarkably elongated pulp-cavities, wliich communicate with the grind- 
 ing surface by means of a narrow fissure. Tlie pulp-cavity is inmie- 
 diately surrounded l)y soft, more or less vascular dentine — the vas<v 
 dentine of Owen — which is covered by a thin layer of unvascular, much 
 
TEETH OF THE VERTEBRATA. 411 
 
 harder dentine. Upon the outside of this comes the cementum, which 
 has a great thickness upon the anterior and posterior face of the tootli. 
 Owing to the unequal powers of resistance Avhich these substances offer, 
 the teeth wear in such a manner as to present two transverse crests each, 
 and are therefore spoken of as tophodont. They are confined to the 
 maxillary and mandibular bones and grow from persistent pulps. 
 
 In another extinct allied genus, Megcdonyx, the teeth are oval in sec- 
 tion, and did not wear into transverse crests as in Megatherium, but have 
 slightly concave grinding faces. Tlie first tooth of the upper sei'ies also 
 is considerably enlarged and caniniform in shape, as in one of the liv- 
 ing sloths. Another nearly-related form is Mylodon, likewise extinct, 
 which exceeded the rhinoceros in size. The first tooth above, instead 
 of being enlarged and caniniform, is smaller than the succeeding ones, 
 and otherwise like them in pattern. 
 
 The dental formula of the three-toed sloth (Bradypus tridactylus) is 
 I. ^, C. •§, M. f Zf = 18. It has been observed, however, that there is 
 in some young examples of this species a small extra tooth in the lower 
 jaw^ just in front of the first permanent one, which is shed before the 
 animal attains to the adult state. The teeth are relatively small, of a 
 columnar form, and implanted to a moderate depth in the substance of 
 the jaws by a deeply-excavated base for the accommodation of the per- 
 sistent pulp. The grinding surface presents a central depression in the 
 vaso-dentine, surrounded by a raised rim on its outer margin composed 
 of the harder dentine, which usually wears unequally into one or two 
 prominent points. The teeth of the upper and lower jaws do not 
 oppose each other exactly, but alternate when the mouth is closed. 
 
 In the two-toed sloth (Choloepus didactylus) the dental formula is the 
 same. The first tooth in each series, which in the edentates generally 
 is the smallest, is here greatly increased in size, of a subtrihedral form, 
 and of a caniniform pattern. They are separated by a considerable 
 diastema from the rest of the teeth, and are implanted above in the 
 maxillary bones a short distance behind the maxillo-premaxillary 
 suture. It will be seen, therefore, that as far as the definition of a 
 canine tooth is concerned, all the conditions are fulfilled ; but the 
 tooth in the lower series, which has undergone a similar modification, 
 violates the definition of a canine, inasmuch as it closes behind the 
 upper caniniform tooth instead of in front of it. It is therefore a 
 matter of uncertainty whether these teeth are strictly homologous with 
 the canines of the diphyodont Mammalia or not. From the manner in 
 which they oppose each other the posterior surface of the upijcr and the 
 anterior surface of the lower are extensively abraded, and their summits 
 worn into sharp points, which would render them efficient ■weapons of 
 offence or defence should the animal choose to use them as such. 
 
 The next tooth of the upper series is relatively small, and is implanted 
 rather obliquely, with the summit inclined backward and inward. The 
 two following teeth are larger, with a central depression upon the grind- 
 ing face, and having the external and internal portions of the rim pro- 
 duced into sharp points. The last tooth is about equal to the second in 
 size, which it also resembles in form. 
 
 The teeth of the lower series resemble those of the upper, with the 
 
412 DENTAL ANATOMY. 
 
 exception that the three posterior ones are more robust and gradually 
 decrease in size from the second to the last. Viewing the teeth and 
 accessory bony structures of this animal as a whole, the premaxillse are 
 remarkable for their small size, little extension anteriorly beyond the 
 maxillse, and the complete absence of the ascending or nasal process, as 
 well as their edentulous condition. The palatal plates of the maxillse 
 are widest in front and gradually narrow posteriorly, causing the dental 
 series of opposite sides to converge behind. In the lower jaw the two 
 halves are completely co-ossified, as in monkeys and man ; the anterior 
 part of the sympliy.sis is produced into a peculiar spout-like termination, 
 at the base of which the jaw widens rapidly ; and the rami are little 
 divaricated posteriorly. The posterior teeth are implanted in a strong 
 inwardly projecting ledge, in consequence of which the dentigerous 
 boi'der gradually approaches the median line as it proceeds backward. 
 The mental foramen is placed below the interval between the third and 
 fourth teeth near the middle of the ramus. 
 
 With respect to the teeth themselves, Owen gives the following com- 
 mon and constant characters of both recent and extinct sloth-like ani- 
 mals, which would include the megatheroids : " Teeth implanted in the 
 maxillary, 'never in the intermaxillary bones; few in number, not ex- 
 ceeding f zf; composed of a large central axis of vascular dentine, 
 with a thin investment of hard or unvai^cular dentine, and a thick outer 
 coating of cement. To these, of course, may be added the dental cha- 
 racters common to the order Bruia — viz. uninterrupted growth of the 
 teeth, and their concomitant implantation by a simple deeply-seated exca- 
 vated base, not separated by a cervix from the exposed summit or crown." 
 
 Of the two Old World genera now living, but one has teeth. This 
 is the aard-vark (Orycteropus), or, as it is sometimes called, the Cape 
 ant-eater. Its dental formula is M. •^Z-g- = 26, of which the anterior 
 ones of each series are not unfrequently wanting or concealed by the 
 gum. The teeth of the superior set progressively increase in size from 
 before backward up to the last tooth, which is smaller. They are oval 
 in section, with the exception of the fourth and fifth, and have wedge- 
 shaped triturating surfaces, like the armadillos. The fourth and fifth 
 above and the last two below have t-wo vertical grooves, one upon each 
 side, which give to them an hour-glass shape upon section. 
 
 The teeth of this animal do not exhibit the customary excavated base 
 of the Edentata generally, but are continued solid to the bottom of the 
 sockets. Their minute structure is peculiar, and resembles that found in 
 Myliobatejs among the elasmobranch fishes ; the dentine is of the variety 
 known as plici-dentine. This consists of a series of small vertical par- 
 allel tubuli whicli pass up from, and are virtually prolongations of the 
 pulp-cavity. From these the dentinal tnbuli radiate toward the per- 
 ipliery, just as tliey do from the single pulp-cavity of the hmuan tooth 
 already described. Owing to this pecidiarity. Prof Owen regarded the 
 tooth of Ori/ctcropHS as an aggregate of many denticles, each with its 
 proper pulp-cavity and dentinal tubes. 
 
 In Europe fossil remains of edentates are known from the Middle 
 Miocene of Sansan in France and the upper INIiocene deposits of Piker- 
 mi in Greece. Two genera, 3Iacrofhcriuia and Ancylothc rium, .ha.\e been 
 
TEETH OF THE VERTEBRATA. 413 
 
 described by Lartet and Gaudry, from feet and limb bones principally, 
 nothing being known of the skull or teeth. In South America fossil 
 remains of this order are very abundant in the Pampean or Pliocene 
 deposits. Older deposits on the Parana River have furnished M. 
 Ameghino with numerous forms which stand in ancestral relation to 
 those of the Pampean beds, and which, it is interesting to observe, 
 have more or less enamel on the teeth. In North America, Prof. 
 Marsh has described a genus under the name of Morotheriun, from 
 the Loup Fork or Upper Miocene strata, from feet bones only. The 
 teeth are not laiown. 
 
 Teeth of the Oetacea. 
 
 According to Dr. Theo. Gill's arrangement, the cetaceans are divisible 
 into three groups or sub-orders, as follows : 3IysHcete, including the 
 " whalebone whales ;" Denticete, or the " toothed whales," and Zeuglo- 
 dontia, a division which includes Zeuglodon and several other extinct 
 genera. In the Mystieete, teeth are present in a foetal state only, being 
 absorbed before birth. This loss of the teeth is compensated for by the 
 development of large corneous plates, the " baleen plates," which depend 
 from the roof of the mouth. The more important of these are of a tri- 
 angular form, and are arranged along each side of the palate in such 
 a manner as to be transverse to the axis of the skull, the centre being 
 occupied by smaller ones, also placed transversely. Altogether, they 
 form by their extremities a vaulted surface into which the large tongue 
 fits accurately, their edges being broken up into numerous stiff hairs 
 which project into the mouth. The animal feeds by taking large quan- 
 tities of water into the mouth and expelling it again through the nos- 
 trils (" spouting ") ; any small aquatic animals which may have been 
 contained in it are entangled in the fringes of the baleen plates, and 
 subsequently coUected by the tongue and swallowed. It will thus be 
 seen that the baleen acts as a sort of sieve or strainer, and is pre- 
 eminently adapted to the capture of the small aquatic forms with which 
 the sea in certain places literally swarms. Each baleen plate possesses 
 a. pulp situated in a cavity at its base, from which it is developed, and 
 through which it is regenerated as fast as worn away. According to 
 Tomes, each hair-like fibre has within its base a vascular filament or 
 papilla; "in fact, each fibre is nothing more than an accumulation of 
 epidermic cells concentrically arranged around a vascular papilla, the 
 latter being enormously elongated. The baleen plate is composed 
 mainly of these fibres, which constitute its frayed-out edges ; and in 
 addition to this there are layers of flat cells binding the whole together 
 and constituting the outer or lamellar portion." 
 
 All the whalebone whales possess rudimentary teeth, or rather dentine 
 and enamel organs, which undergo very little calcification before absorp- 
 tion sets in. In the fin-backs (Bakenoptei-a) these dentine organs are 
 simple in the front part of the mouth, bifid in the middle, and trifid in 
 the back part of the jaw. They are placed in an open groove along the 
 jaw, as in all other Mammalia at this stage of embryonic growth, and 
 do not differ from them in any important particular. 
 
414 DENTAL ANATOMY. 
 
 In the second sub-order (Denticete) no baleen plates are develofied ; 
 teeth are always jDresent, and are more or less persistent. They are 
 imjDlanted by single roots, and are in some instances very numerous. 
 No second dentition has ever been observed in any member of this 
 group, and they are, so far as known, truly monophyodont. In the 
 common porpoise of our coast (^Delphinus chymenc), which is an average 
 example of this sub-order, the teeth are about ninety-four in number, 
 and are lodged in the pi-emaxillary, maxillary, and mandibular bones. 
 They are implanted by single slightly enlarged fangs in ill-defined 
 sockets incompletely partitioned off from each other, and in what at 
 first sight seems to be a wide-open groove. Their crowns taper grad- 
 ually to a sharp point, which is strongly incurved. The first t\\o teeth 
 in the upper jaAv ai'e small and implanted in the premaxillary bone, 
 "which furnishes a very small part of the dentigerous border of the ujiper 
 jaw. Behind these the maxillary teeth rapidly increase in size up to the 
 seventh or eighth tooth, after which they continue to the fifteenth or six- 
 teenth almost ec^ual in size, and then gradually diminish in size toward 
 the jjosterior part of the jaw. The teeth of the inferior series are like 
 those of the ujDper, except that the posterior ones are more robust. 
 The ja\\s are remarkable for their great length and narrowness, and the 
 arrangement of the bones of the face A\-hen compared with other mam- 
 mals is also peculiar. The coronoid process of the lower jaw is i il isolete. 
 
 In other members of the Delphinida' — the dolphin, for example — the 
 teeth are often as many two hundred, the greatest numljer exhibited by 
 any mammal, or they may be reduced to a single functional tooth, as in 
 the uar\vhal [Monodon). In this latter species four teeth are found in a 
 foetal state, but the two lateral ones are lost or alisrirbed before birth. 
 In the male nar-s^hal the left of the t^vo anterior ones, ^^•hich is placed 
 in the premaxillary bone, grows from a jjersistent pulp and attains a 
 length of ten or twelve feet. This formidable tusk is almost straight, 
 and is marked by sj^iral ridges which wind forwai'd from left to right. 
 The corresjjonding tooth of the opposite side sometimes reaches a devel- 
 opment equal to that of the left, but more frecpientlv its growth is 
 arrested and it remains buried beneath the gum, as do both in the 
 female. Owing to this circumstance, it has been thought that it 
 serves as a sexual weapon similar to the antlers of the deer, but until 
 the habits of the animal are better known this explanation of its use 
 must remain conjectural. 
 
 The great bottle-nosed whale {Hyperobdon hiden^) is, to all outward 
 appearances, edentulous, but careful examination reveals the presence of 
 two, sometimes four, well-calcified conical teeth in the front part of the 
 jaA\', M'hich remain more or less completely hidden by the gum. In 
 addition to these, there are usually twelve or thirteen small rudimentary 
 teeth imbedded in the gums of both jaws, which soon disappear. 
 
 In the sperm whale (Pliyseter macrocephalus) the exposed and func- 
 tional teeth are i'(mfined to the lower jaw. These are ajjout twenty- 
 seven in number in each ramus, loosely implanted in a AA'i de-open gutter, 
 with the alveoli or sockets scarcely perceptible. They are at first 
 sharply conical, but by attrition -wear down into obtuse cones, biting 
 into pits or cavities in the gums of the upper jaw. In this jaw there 
 
TEETH OF THE VERTEBBATA. 415 
 
 are a number of persistent rudimentary teeth concealed in the tliick 
 gums, one pair of which is exposed in the small pug-nosed sperm whale 
 (P. simus). 
 
 In the dolphin of the Ganges {Platynista gangetivn) the total number 
 of teeth is one hundred and twenty-four, of which there are thirty upon 
 each side above and thirty-two upon each side below. In the young 
 animal their crowns are produced into sharp cones, but by attrition the 
 posterior teeth are worn down to such an extent as to become molari- 
 form in shape. The last tooth in this species not unfrequently develops 
 a double root or fang, and is the only example of the kind to be met 
 with in living cetaceans. 
 
 The teeth of the dolphins of the Amazon are surrounded at the bases 
 of their crowns by a well-marked ledge or cingulum, -which throws up 
 a strong internal cusp. On this account Dr. Gill elevates the genus 
 Inia to the rank of a family. 
 
 The sub-order Zeuglodontia is extinct, and includes a number of fossil 
 cetaceans, some of which are estimated to have attained 'a length of 
 seventy feet. They differ from living representatives of the order in 
 many important osteological characters, but not more prominently than 
 in their dental organization. Besides being heterodont and ha\ang the 
 posterior teeth implanted by two or three roots, some, if not all, were 
 diphyodont as well. The exact extent of the replacement, however, is 
 not fully known, but it is certainly true that two sets of teeth were 
 developed. In Zeuglodon eetoides (Fig. 199), from the Claiborne 
 
 Fig. 199. 
 
 Side View of the Skull of Zeuglodon eetoides (after Gaudry). 
 
 Eocene deposits of Louisiana, Alabama, and Mississippi, the teeth 
 are divisible by their form and position into incisors, canines, and 
 molars. Three teeth with conical recurved crowns are implanted by 
 single roots in each premaxillary bone, which in this animal contributes 
 a considerable part of the tooth-bearing border of the upper jaw. Of 
 these the anterior is the smallest and placed at some distance behind the 
 extreme anterior border, the posterior ones gradually increasing in size. 
 Behind these, near the maxillo-premaxillary suture, is a strong recurved 
 single-fanged tooth of a caniniform pattern, and one which both by 
 position and form becomes the homologue of the canine in the ordinary 
 heterodont dentition. The rest of the alveolar border is occupied by 
 four rather large more or less trenchant teeth, referable to the molar 
 and premolar series. Each of these is implanted by two roots, and has 
 a laterally compressed crown of a triangular form with the apex of the 
 triangle at the summit. Each of the anterior and posterior edges are 
 
416 DENTAL ANATOMY. 
 
 interrupted by three well-marked cusps, which give to the tooth a 
 strongly-serrated appearance. 
 
 The heterodont and diphyodont character of the teeth of this cetacean 
 serves to bring the anomalous, and in many respects degenerate, dental 
 organs of this order into the closest relationship witli the teeth of the 
 ordinary diphyodont Mammalia, and, being the oldest member of 
 the group so far known, goes far toward filling the wide gap between 
 these aquatic and the terrestrial mammals. 
 
 Teeth of the Unguiculate Series. 
 
 In considering the dental organization of this vast assemblage of 
 mammiferous animals it is necessary to have at the very outset a correct 
 conceptiou of the primitive or ancestral stock from which all of them 
 have been derived, if such can be found to exist. This can be learned 
 only by a careful study of the successional history of tlie various orders 
 composing it. In searching, then, for this original stem we can by this 
 method exclude many of the groups from this position by fixing the 
 date of their appearance, and thereby establishing their exact limit in 
 time. We know, for example, that the Primates could not be the 
 ancestral group, for the obvious reason that they do not extend beyond 
 Miocene time ; nor the Carnivora, which appeared about the same time ; 
 nor the Rodentia, which date from the Middle Eocene ; nor the Cheir- 
 optera, which can be traced back no further than the Upper Eocene. 
 We are therefore restricted in our choice to the insectivores, lemuroids, 
 creodonts, or tillodonts, which alone of the entire series continue back- 
 ward to the base of the Eocene Period. With reference to the creodonts, 
 I do not believe that any important distinctions exist between them and 
 the insectivores, while the line between tliis latter group and the lem- 
 uroids and tillodonts becomes extremely shadowy at this point. 
 
 Prof. Cope unites the insectivores, lemuroids, tillodonts, creodonts, 
 and tseniodonts into one order, whioli he calls Bunothcria, and defines 
 the several sub-orders as follows : ' 
 
 I. Incisor teeth growing from persistent pulps : 
 
 (o) Canines also growing from less persistent pulps, agreeing with external 
 incisors in liaving molariform crowns .... Tcerdodonta. 
 
 (b) Canines rudimental or wanting ; hallux not opposable . . . Tillodonkt. 
 
 (c) Canines none ; hallux opposable . . . Daubenlonioidea. 
 II. Incisor teeth not growing from persistent pulps : 
 
 (n) Superior true molars quadritubercular ; hallux opposable . . Prosimim. 
 (h) Superior true molars quadritubercular ; hallux not opposable. Insectivora. 
 (c) Superior true molars tritubercular or bitubercular; hallux not opposable. 
 
 CrtDdonta. 
 
 I believe, M-ith this author, in classifying those forms in which the 
 incisors grow from ])ersi.stent pulps as a di.stinct group from those in 
 which the incisors are normal, as far at least as their o;rowth is con- 
 cerned. In the first division there are thi-ce well-defined sub-orders. In 
 the second division it is extremely questionable whether more than two 
 sub-orders should 1 )e made. If we use the opposable and non-opposable 
 condition of the hallux as a character, we Mill have two perfectly natural 
 
 ' Proeeeding.i Academy Natural Sciences Philada., 1883. 
 
TEETH OF THE VEBTEBRATA. 417 
 
 series — the prosimian or lemuroid and tlie insectivorous ; but if we go 
 further, and establish another sub-order upon tlie tritubercular or quad- 
 ritubercular character of the superior molar teeth, it will necessitate the 
 wide separation of forms closely related to each other l)y every import- 
 ant feature of their anatomical structure — a course which I do not deem 
 advisable nor in keeping with our present knowledge of the subject. 
 
 According to Prof. Cope's definition, the only character in which the 
 Creodonta differ from the Insectivora is the tritubercular superior molars 
 as distinguished from the quadritubercular ; and in order to make the 
 Creodonta homogeneous he is compelled to take out of the old group 
 Insectivora the Tunpaiadw, Centetidce, Chrysiochloridw, and Talpidw, and 
 place them in the Creodonta. Aside from the inadvisability of such a 
 course, these teeth in many of the above-named families are altogether 
 intermediate between the tritubercular and quadritubercular pattern, the 
 postero-internal tubercle being represented often by a rudimentary cingu- 
 lum, which may be entirely absent or produced into a strong cusp. 
 Then, again, the superior molar teeth of the prosimian division are 
 indifferently tritubercular or qHadritubereular ; and if we adhere to 
 this practice in the one, why not in the other? In consequence of 
 these facts, I propose to unite the Creodonta with the Insectivora into 
 a single division, for which the old name Insectivora may be retained. 
 
 Thus constituted, palseontological history, in my judgment, points 
 strongly to the fact that this group stands in the important relationship 
 of ancestors to a large part, if not the whole, of the unguiculate IVTani- 
 raalia. Working upon this hypothesis, it will be desirable to describe 
 the more important types of dental structure to be met with in this sub- 
 order, after which they can be followed out to their respective termina- 
 tions in the various orders and sub-orders which make up the series. 
 
 Insectivora. — The simplest form of dental structure in this sub-order 
 is exhibited by the extinct genera llcsoiii/x and Dissacus of Cope, from 
 the American Eocene strata. The teeth of Jlesom/x (Figs. 200, 201) 
 are forty-four in number, disposed as follows : I. f , C. ]-, Pm. f , M. f 
 = 42. The incisors are relatively small, with subconic crowns, which 
 are closely approximated. The superior canines are large, recurved, and 
 pointed, being placed at a considerable distance from the incisors to 
 accommodate the crown of the inferior canine. The three anterior pre- 
 molars of the upper jaw are two-rooted, with the exception of the first, 
 which is probably single-rooted. They have comparatively simple com- 
 pressed crowns, with a principal cusp and a posterior basal lobe, sur- 
 rounded by a basal cingulum. The fourth is more complex, and resem- 
 bles the true molars posterior to it. Like them, it has three principal 
 cusps, of which two are external and one internal, giving to the crown 
 a triangular shape. In the first true molar the postero-external angle 
 of the crown is produced into a strong blade-like process, a develop- 
 ment of the cingulum which is conspicuous in all. The last molar of 
 this series is bicuspid, the posterior of the two external cusps being 
 absent. 
 
 In the lower jaw both the premolars and molars are remarkable for 
 their simplicity. The first premolar is smgle-rooted, and has a sub- 
 conic crown, as in the dog. The teeth behind it are two-rooted, and 
 
 Vol. I.— 27 
 
41.'^ 
 
 DENTAL ANATOMY. 
 
 have a general premolariform appearance, the true molars exhibiting but 
 little departure from the conical pattern of the lower Vertebrata. As in 
 the premolars of the dog, their crowns are laterally compressed, of a 
 
 Fig. 200. 
 
 ii \ 1 I I I I I 
 
 third natural size (after Cope). 
 
 triangular form when viewed from the side, having a principal median 
 cusp, to which are added an anterior and posterior smaller one from the 
 cingulum. 
 
 It is a matter of considerable interest to find in this ancient represen- 
 tative of the unguiculate series so simple and generalized a dentition, 
 inasmuch as it furnishes a key to an interpretation of the lobes and 
 cusps of the teeth of many of the succeeding forms. It is more than 
 probable that this particular species is not the original ancestral form 
 from which the (ithers have been derived, on account of certain charac- 
 ters presented l)y the skeleton, but, as far as the teeth of the lower jaw 
 are concerned, they exhibit just such a transitional condition between 
 the primitive cone of the theromorph Reptilia and the lowest forms 
 of mammalian teeth as we would most reasonably expect to find in 
 the pi'imitive ancestor. 
 
 The various steps in this process of dental evolution I conceive to 
 have been as follows : (1) additions to the anterior and posterior edges 
 of the cone and the formation of a cingulum ; (2) division of the single 
 
TEETH OF THE VERTEBBATA. 
 
 419 
 
 root into two ; (3) addition of basal cusps from the cingulum. It is a 
 fact worthy of notice that in the conical dentition the teeth of one series 
 do not exactly oppose those of the other, but close in the intervals be- 
 tween them. This in animals that attempted to crush a morsel of food 
 would cause stimulation of the anterior and posterior edges of the tooth, 
 thereby determining the point of the greatest nutritive activity and (ton- 
 sequent growth. Long-continued vertical pressure I believe to be an 
 adequate cause for the appearance of the wrinkle or fold of the enamel 
 covering at the base of the tooth which is designated as the cingulum. 
 
 Fio. 201. 
 
 Skull of Mesonyx ossifragus^ anterior to post-glenoid process, one-third natural size, from the Wasatch 
 beds of Wyoming (after Cope). 
 
 The formation of two roots I believe to have been the result of the in- 
 equality of pressure exerted upon each tooth during the act of mastication, 
 whereby there was an effort to displace the tooth in an antero-posterior 
 direction, or, in other words, to give it a forward and backward rocking 
 
420 
 
 DENTAL ANATOMY. 
 
 movement, as the greatest pressure \\-as in front of or behind it. This 
 would cause the stimulation of the anterior and posterior faces of the root, 
 and as a i'(;nse(]^uence of this a vertical groove was first formed upon each 
 side, -which eventually coalesced, dividing the root into two. As we 
 have already seen, this condition is found in a theromorph reptile, 
 and is likewise to be found in the premolars of many existing animals. 
 The development of basal cus])s would naturally follow at those points 
 where the crown sustained the greatest amount of resistance, which 
 would be at the base of the triangle. 
 
 It is a rule of pretty general apjjlication in heterodont teeth that the 
 molars are more modified than the premolars. This, in all jjrobability, 
 results from the greater mechanical advantage Mhich is gained by bring- 
 ing the morsel to be crushed or divided to the posterior part of the 
 mouth ; that is to say, the resistance as near to the po^ver as possible. 
 The power in this case is the muscles which close the mouth, which, 
 being attached to the posterior part of the jaw, exert the greatest 
 influence upon those teeth in the vicinity of their attachment. 
 
 The next step in dental complication is seen in the genus Dissacus, 
 from the lowest Eocene, the lower teeth of which are represented in 
 Fig. 202. They are very similar in general appearance to those of 
 
 FiCx. 202. 
 
 Dissacus navfijwms, Cope, r.ight :\randibular Ramu«, three-fourths natural tize ; a, external ; 6, supe- 
 rior view, from the i^uerco Beds of New Mexico (after Cope). 
 
 Mesonyx, with 'iNhich they alsd agree in number. There is, however, an 
 additional cusp developed upon the inner side of the median cone near 
 its summit, which is the homologue of the internal tubercle of the infe- 
 rior sectorial of the dog, as well as that of many other animals of the 
 unguiculate series. The upper teeth are not known. The genera in 
 which the mandibular teeth jiresent this premolariform structure are 
 associated hy Cope into a family M'hich he calls the 3Tcm)n/fJild(r. 
 
 Asa probable derivative of this family we have the t'xtinct family 
 Hi/anodontidce, of which the teeth of the single genus Hycvnodon are 
 represented in Fig. 203. This animal is known, so far, from the jNIio- 
 cene de])osits of this country and Europe only, and has been shown by 
 Prof. V^\ B. iScott of Princeton College to be a near relative of Jlemny.r. 
 The dental formula is, according to Gaudry, I. |, C. l, Pm. +, M. i 
 = 42. The incisors resemble those of the dog, the median pair being 
 
TEETH OF THE VERTEBRATA. 
 
 421 
 
 the smallest, the outer pair the largest. The canines are large, pointed, 
 and recurved. The anterior premolars above are two-rooted and have 
 premolariform crowns. The third and fourth are three-rooted, with 
 
 Fig. 203. 
 
 HycBnodon horridus^ Leidy. Skull, one-half natural size (from Cope, after Leidy). 
 
 three external and one internal cusp, which in the third premolar is 
 small and placed far back ; in the fourth it is large and has a position 
 nearer the middle of the tooth ; while in the first and second molars it 
 is anterior and more or less rudimental. The anterior cusp and median 
 
422 
 
 DENTAL ANATOMY. 
 
 cone in these latter teeth form cutting blades and are truly sectorial in 
 their nature. 
 
 In the lower jaw the first premolar is t\v'o-rooted, with a compressed 
 crown without basal lobes. The second, third, and fourth have well- 
 developed posterior basal lobes, -with the anterior absent. The first true 
 molar lias three lobes, which form an imperfect trilobed sectorial blade. 
 The second is also trilobed, having the anterior cusp and median cone 
 developed into a true sectorial, while the posterior lobe forms a cutting 
 heel. In the third the heel is rudimental or absent, and the anterior 
 lobe and median cone are modified into a perfect trenchant blade. 
 
 From the Meson y chid ir. we jtass, through Dksacus and Ttiimdon, to 
 another extinct family, '\Nhich Prof. Cope calls the LejjtictkJw. In an 
 
 Fig. 204. 
 
 Skull and Part of tlie Posterior I'oc.t of lu-o individuals of Sh/poloplim ivhltia. Cope, two-thirds nat- 
 ural size: a,b, side and under views of tlie slcull; c, portion of lower jaw; il, ankle-joint (after 
 Cope). 
 
 Eocene genus of this family, Sti/poloplms, Cope (Fig. 
 ■ " f , C. 1 Pm. A, M. t = 44. The " ' 
 
 204), the dental 
 formula is I. |, C. l Pm. f, M. f = 44. The incisors, canines, and 
 premolars are very like the corresponding teeth in Mesonyx. The last 
 superior premolar is, like the true molars, tritubercular, with the pos- 
 
TEETH OF THE VERTEBBATA. 
 
 423 
 
 terior external angle produced into a prominent process, which is con- 
 nected with the postero-external cusp by a sharp cutting ridge. The 
 cingulum also furnishes a broad ledge upon the outside of the two 
 external cusps. 
 
 In the three inferior true molars the median cone, the anterior basal 
 lobe, and the internal tubercle are all well developed, and are disposed 
 in such a manner as to form an equilateral triangle, with the apex 
 directed forward and the base backward. Of these, the anterior basal 
 lobe occupies a position at the apex of the triangle, the median cone 
 and internal lobe being placed at the external and internal angles of 
 the base respectively. The posterior basal lobe is also present in the 
 form of a low heel, which may in some genera retain a simple cutting 
 form or may be broken up into several and become " basin-shaped." 
 This form of tooth Prof. Cope proposed some years ago to designate by 
 the name of " titbereulo-sedorial." There can be little doubt that it 
 furnishes the point of departure for the sectorial teeth of the lower jaw 
 
 Fig. 205. 
 
 
 
 Left Mandibular Ramus of Triisodon quivirensix, three-fourths natural size, from the Puerco of New 
 Mexico; «, external view, displavinglast temporary molar in place; ft, the same from above; c, the 
 same, internal side, the temporary molar removed and the permanent fourth premolar displayed 
 in the jaw ; (/, the fourth premolar seen from above (after Cope). 
 
 of the modern Carnivora on the one hand, and the quadritubercular 
 lower molar of the entire unguiculate series on the other. 
 
 As will be seen, it displays the same elements that are found in the 
 inferior sectorial of the dog, the only difference being in the relatively 
 
424 
 
 DENTAL ANATOMY. 
 
 smaller size of the internal tubercle and the modification of the primi- 
 tive cone and anterior basal lobe into a more perfect sectorial form in 
 the dog. The quadritubercular tooth has been derived from this by the 
 suppression of the anterior basal lobe, the reduction in size of the median 
 cone and internal tubercle, and the division of the heel into two cusps, 
 whereby the median cone becomes the antero-external tubercle, the 
 internal tubercle the antero-internal, and the heel the two posterior 
 ones. 
 
 Tlie genus Trumdon of Cope (Fig. 2(_)5) affords a perfect transition 
 between titypolaphus and Uwsdeiis, as far as the pattern of the infe- 
 rior teeth is concerned. In another genus, Chriacus, Cope, from 
 the LoAver Eocene, which is provisional!}' referred to this family, the 
 upper molars are tritubercular \\'ith a strong internal cingulum, -which 
 develops a rudimental fourth cusp behind. This forms one of the 
 examples referred to in which it is difficult to say whether the teeth 
 in question are tritubercular or quadritubercular, and goes far toward 
 invalidating the definition of the (JrcodonUi as given l)y Cope. 
 This author says in reference to this family, and more especially to 
 this genus : " Two groups are easily recognized among the Lcptirtida;. 
 In the first of these the last or fourth inferior premolar is a simple pre- 
 molariform tooth, different from the inferior true molars and without 
 
 any internal cusp. In the second 
 division the fourth inferior premo- 
 lar is either like the first true mo- 
 lars or approximates their form by 
 the presence of an internal tubercle. 
 To the latter group belongs the ge- 
 nus C7i/'(Vrr!(,s, which, from the slight 
 development of the fourth inferior 
 premolar, approximates the first di- 
 vision. This genus may, however, 
 be improperly referred to the Creo- 
 donta." ' 
 
 Still another genus of this family, 
 iliockmua, als(.i from the Eocene, 
 presents truly quadritubercular 
 lower molars. The premolars are 
 simple and conical, and differ widely 
 in their structure from the molars. 
 The superior true molars are sim- 
 ilar to those of the preceding genus, 
 with the exception that the fourth 
 tubercle is better defined and fur- 
 nishes another example of the tran- 
 sition between the tritubercular and 
 quadritubercular condition. 
 
 In the typical genus Leptidis of 
 Leidy (Fig. 206) the dental formula is probably the same as that of 
 "■ ipolophus, the lower jaw being imperfectly known. The upper teeth 
 
 ^ " The Creodonta," Jmerican NaluralisI, Aytril, 1884, p. 348. 
 
 Leptidis haydpni, Leidy. Skull, natural size, from 
 the White River beds of Nebraska i from Cope, 
 after Leidy). 
 
TEETH OF THE VERTEBRATA. 
 
 425 
 
 are like those of Stypolophus in having the fourth premolar and all the 
 true molars tritubercular. There is no broad ledge external to the outer 
 cusps, however, as in that genus, and the posterior external angle of tlie 
 crown is not produced. It is from the White River Miocene of this 
 country, as is also a nearly-related genus, Jcfojss of Cope. The only 
 difference between these two genera is the more complex form of the 
 fourth superior premolar in the latter. 
 
 The living genus Centetes, or tenrec of Madagascar, is closely related 
 to this family, and differs only in the incomplete condition of the zygo- 
 matic arch. The number and form of the teeth are very like those of 
 Leptlditi, and it is highly probable, as Cope suggests, that Ccutetes is 
 the living descendant of this genus. 
 
 Another quite remarkable genus which Cope places in this family is 
 from the Eocene, and was 
 
 described by him under the Fig- 207. 
 
 name of Usthonyx. Its 
 dental formula (Fig. 207) 
 is I. i C. 1, Pm. I, M. f 
 = 34. The single superior 
 incisor of each side is great- 
 ly enlarged, and exceeds the 
 canine in size. The first pre- 
 molar is small and has a 
 simple crown. The next is 
 larger, and is tritubercular. 
 The third is like the true 
 molars, with the exception 
 that it lacks the internal 
 cingulum. The true molars 
 have two external cusps, 
 bordered upon the outside 
 by a broad ledge which is 
 produced anteriorly into a 
 marginal cusp. There is 
 a large internal cusp, from 
 which is developed at its 
 inner posterior extremity 
 a strong cingulum, the representative of the fourth cusp, which is con- 
 tinued thence around the base of the crown behind to join its broad 
 external portion. This is another case wherein the tritubercular and 
 quadritubercular question is involved in uncertainty. 
 
 In the lower jaw the median incisors are small, the outer pair 
 enlarged, almost equalling the canines. The first two premolars are 
 small, the third larger, resembling the true molars somewhat in form. 
 The molars support two Vs, of which the anterior is most elevated. 
 An analysis of the crown shows it to be of a modified tuberculo-secto- 
 rial nature, wherein the three anterior cusps are connected by ridges 
 that extend quite to their summit and form the anterior V. The broad 
 heel displays two cusps connected by a strong ridge ; from the outer of 
 these, again, another ridge passes obliquely forward to join the internal 
 
 Esthonyz hurmeisteri, Cope ; a, 6, c, parts of upper and lower 
 jaws, two-thirds natural size (after Cope). 
 
426 
 
 DENTAL ANATOMY. 
 
 cusp, thereby completing the second V. The last molar has a fifth cusp 
 behind, in this respect resembling many of the lemuroids. 
 
 The family most nearly ajjproximated to this genus is that including 
 the shrews {8orici(hr), which always have two incisors both above and 
 below, greatly enlarged. In Blarina talpoides, a living species of this 
 country, the teeth (Figs. 208, 209) are thirty-two in number, of which 
 
 Fig. 208. 
 
 Fig. 209. 
 
 Vertical View of Grinding 
 Side View of a Portion of a Skull of Blarina talpoides (much enlarged). Surface of a, a lower mo- 
 
 The sixth tooth of the upper series is placed somewhat internal to lar, and b, an upper mo- 
 
 the others, and is not repiesented in the drawing. lar (enlarged.) 
 
 twenty belong to the upper and twelve to the lower jaw. Owing to the 
 very early co-o.ssification of the jiremaxillse with the maxillfe and the 
 paucity of suitable material, I am at present unable to give the proper 
 dental formula of this animal. Neither can I find any statement upon 
 the subject further than that of Owen, in which he refers to the European 
 s])eeie,s,-/S'o/-c.r (iraiHcuK, with only eight teeth upon each side in the upper 
 jaw, and says : ' " The determination of the small teeth between the large 
 anterior incisftrs and the multicu.spid miliars depends upon the extent of 
 the early ankylosed intermaxillaries ; the incisors being defined by their 
 implantation in these bones, the succeeding small and simple crowned 
 molars must be regarded as premolars, not any of them having the 
 development or office of a canine tooth : their analogues in the lower 
 jaw are implanted by two roots." If he confines his statement to this 
 species, it is probably correct to refer all those teeth between the large 
 incLsors and the anterior one of the last three to the premolar scries; but 
 in Bkirinn there are six teeth to be disposed of. AlloM'ing the normal 
 number of premolars (four). Me have either three incisors and no canine, 
 or, as is most probable, two incisors and a canine, M'hich would give the 
 following formula: I. f , C. ^, Pm. f , M. |-. This determination of 
 course may prove to be incorrect. 
 
 ^ Odontoyraphy, p. 418. 
 
TEETH OF THE VERTEBRATA. 427 
 
 The two large incisors above are hook-shaped, with a prominent pos- 
 tei-ior ledge at the base, which in some species of this family is produced 
 into a strong basal cusp. The next two teeth are much smaller and sub- 
 equal, while the three following, rapidly decrease in size, the last becom- 
 ing very small. Exclusive of the large hook-shaped incisor, the above- 
 mentioned teeth display a principal cone with an internal lobe and 
 an external cingulum, which is most distinct in the fourth, fifth, and 
 sixth. The next tooth, which I take to be the last premolar, marks an 
 abrupt change both in the character and size of the teeth of the ujjper 
 series. It is almost equal in size to the first true molar, which it resem- 
 bles very closely in structure. 
 
 The crowns of the molars may be described as consisting of four 
 principal cusps or tubercles, of which two are external and two internal, 
 and are therefore quadritubercular. The two external cusps stand at the 
 apex of two Vs, which open externally, giving to this part of the tooth a 
 distinct W pattern. At the extreme antero-external angle of the crown 
 there is a considerable cingular cusp, which is connected with the main 
 antero-external tubercle by a prominent ridge, thereby forming the first 
 downward stroke of the W. From this main tubercle another well- 
 marked ridge passes outward and backward to another small cusp of 
 the cingulum, situated at a point midway between the two main exter- 
 nal tubercles on the outer edge of the crown, forming the first upward 
 stroke of the W and completing the first V. The second downward 
 stroke of the W is furnished by a ridge connecting the small median 
 marginal cusp with the postero-external tubercle, while the second upward 
 stroke of the W is formed by a ridge continued outward and backward 
 to the postero-external angle of the crown, where it terminates in as light 
 enlargement. 
 
 From the apex of each of the Vs a high ridge passes inward, meeting 
 at the antero-internal angle of the crown, forming a distinct U. The 
 inner part of this crest is the antero-internal tubercle. The isostero- 
 internal tubercle stands just behind it, and exhibits a somewhat cres- 
 centiform pattern. It will be seen that the tooth just described does 
 not differ materially from those of some other insectivores already 
 noticed, especially Esthonyx. The principal differences are to be found 
 in the greater development of the marginal cingular cusps and connect- 
 ing ridges upon the external part of the crown, which we have, in a 
 measure, foreshadowed in Esthonyx and others. 
 
 In the lower jaw the single pair of incisors are large, scalpriform, and 
 procumbent. The two succeeding premolars are small, single-fanged, 
 and have simple crowns. The first two true molars are the largest of 
 the molar and premolar series, and exhibit a structure identical with 
 that of Esthonyx. The last is very small, and corresponds with the last 
 tooth of the upper jaw, which frequently disappears. The crowns of 
 all the teeth are stained a deep wine-color by a pigment which pene- 
 trates the substance of the enamel, this tissue being remarkable for its 
 thickness in all the Insectivores. 
 
 Considerable discussion has taken place in regard to the nature of the 
 external Vs and the exact homology of the two external tubercles. Since 
 this W-structure is common to the superior true molars of all the moles. 
 
428 DENTAL ANATOMY. 
 
 shrews, and insectivorous bats, as well as some others of the unguiculate 
 series, it is desirable to have a thorough understanding of it. Cope 
 maintains ' that the median and anterior marginal cusps are the homo- 
 logues of the two external tubercles of the teeth of such a form as Sty- 
 polophus (Fig. 204), and that the two cusps, which are here homolo- 
 gized as the representatives of the two external tubercles of this genus, 
 he proposes to call intermediate tubercles. INIivart, on the other hand, 
 holds ^ that all the marginal cusps are developed from the cingulum, and 
 that the true external tubercles have come to occupy a more and more 
 internal position on the crown — a view which I believe to be correct. 
 
 The evidence upon which I base my opinion is to be found by exam- 
 ining the teeth of such genera as Siypolophus, Edhonyx, and Scapanus 
 of the unguiculate series, and Thylac'mus, Didelpihys, Fhascogale, and 
 Dasyurus among the marsupials. In the genera StypoIop>hus and Estho- 
 nyx, as we have already seen, the way is paved, so to speak, for the 
 formation of the two Vs by the appearance of a broad ledge external to 
 the two main outer cusps and the elevation of the cingulum into a small 
 cusp at the antero-external angle of the crown, as well as the backward 
 prolongation of the postero-external angle and its connection with the 
 postero-exterual tubercle by a strong ridge. This latter ridge I 
 regard as the strict homologue of the last upward stroke of the W. In 
 these two genera the only modifications necessary to produce the W 
 ^v•ould be greater separation of the two external tubercles and the pres- 
 ence of a median marginal cusp connected with them by ridges. 
 
 In the genus Scapanus, or hairy-tailed moles, of this country the fourth 
 superior premolar does not exhibit the W-shaped arrangement in the 
 same perfection that the true molars do, the anterior 
 V being rudimental or absent. The cusp at the an- 
 tero-external angle, however, ]s present, and can be 
 clearly shown to be of a cingular origin. It cannot 
 therefore, as Cope supposes, represent the true antero- 
 external tubercle, in this tooth at least. In the sec- 
 ond unworn true molar of Dasyurus (Fig. 210) all 
 the marginal cnsps are present, but the median one 
 is not connected with the two main external tuber- 
 cles by ridges, leaving the W imperfect. In this 
 View of the Griuding tooth nothing is morc apparent than the cingular 
 
 Surlace of an Dn- . . n 77 .i • 1 i 
 
 worn Molar Tooth of ongm ot all the margmai cusps ; and no one can 
 ^"iDte7nai;"i,!"elte: doubt, it appears to me, that they are strictly homol- 
 °K'h''crawn°' ''^^^°* ogous with the cusps in a like position in the molars 
 of those animals in which the W is perfectly formed. 
 A careful consideration of the teeth of the genera above mentioned in 
 my judgment effectually disposes of the whole question, and demon- 
 strates beyond doubt the correctness of the position here maintained, 
 notwithstanding the conclusions of so high an authority as Prof. Cope 
 to the contrary. 
 
 Galeopitheous, or the so-called flying lemur, constituting another 
 
 ' "Mutual Relations of the Bunotherian Mammalia," Proceed. Acad. Nat. Sciences 
 Philadelphia, 1883, pp. 81-83. 
 
 ^ Journal of Anatomy and Physiology, ii. 138, figures, 1868. 
 
TEETH OF THE VERTEBBATA. 
 
 429 
 
 Fig. 211. 
 
 Two Incisors of the Lower 
 Jaw of Gtdeopithecus, ex- 
 ternal view (enlarged). 
 
 family (Galeopithecidce), is quite aberrant in the form of its incisors 
 and some of the premolars. The incisors are two in number upon 
 each side in the upper ja\v, and those of the opposite sides are separated 
 by a wide edentulous space ; the first is minute and comparatively 
 simple ; the second is relatively large, two-rooted, and in every way 
 similar to the tooth behind it, which is lodged in the maxillary bone. 
 The form of the crown is that of a greatly flattened cone with anterior 
 and posterior cutting edges. The anterior edge is interrupted by one 
 minor denticle, the posterior by four, making it dis- 
 tinctly serrated. The next tooth behind this one is 
 sometimes called a canine, but it is more probably 
 a premolar ; if this be the case the premolars are 
 three in the upper jaw. The last premolar is like 
 the molars, with three principal cusps and two small 
 intermediate ones. 
 
 In the lower jaw the incisors (Fig. 211) form a 
 continuous arch around the alveolar margin, and 
 are of a most remarkable pattern. They are four in 
 all, of which the outer pair is somewhat the larger ; 
 they have broad incisive crowns, which are cleft to 
 the base by deep vertical fissures like the teeth of a comb. In the 
 middle pair there are seven such fissures, dividing off eight slender col- 
 umns, whereas in the lateral pair there are ten. 
 
 The next tooth has a somewhat similar shape, but there are only four 
 fissures, which do not penetrate so deeply ; its crown cannot therefore be 
 said to be more than serrate. The true molars are quintitubercular, 
 with very elevated cusps. 
 
 In the European mole {Talpa europea), which may be taken as a fair 
 representative of the family Talpickv, the dental formula is I. f, C \, 
 Pm. I", M. -I = 44. The incisors of the upper series are normal both in 
 size and structure. The upper canine is large, recurved, and pointed, 
 and exhibits the remarkable peculiarity of being implanted by two 
 fangs. The premolars are simple compressed 
 teeth, increasing progressively in size from the 
 first to the fourth. The true molars are tritu- 
 bercular, with the W-shaped structure externally. 
 
 In the lower jaw the first four teeth are 
 small and incisiform; the next is large, two- 
 rooted, and caniniform, performing the func- 
 tion of the inferior canine. It is, however, 
 really a premolar, since it closes behind the 
 superior canine, and not in front of it. The 
 tooth immediately in front of it is the true 
 canine, notwithstanding its small size and in- 
 cisive office. The three succeeding premolars 
 are similar to the corresponding teeth above. 
 The true molars are quadritubercular, or rather 
 intermediate between the tuberculo-sectorial and 
 the quadritubercular patterns. 
 
 In the hedge-hogs [Erinacidce) (Fig. 212) and the elephant shrews 
 
 Fig. 212. 
 
 Vertical View of a, the upper 
 jaw, and 6, the lOT^er jaw, of 
 European hedge-hog {Erina- 
 ceus). 
 
430 
 
 DENTAL ANATOMY. 
 
 (Macroscelidcc) the molars are quadritubercular both above and below, 
 and exhibit no traces whatever of the complex W-structure. If it is 
 imperative to make any division of the Insectivora upon the characters 
 of the teeth, I would suggest that the W-arrangement of the cusps 
 of the superior true molars be considered as available for the purpose, 
 although I would be seriously disposed to question this character alone 
 as indicative of community of descent. 
 
 Another family of the Insectivom which in all probability stands in 
 ancestral relationship to the Ou-nkom is the one which Cope calls the 
 Jliacidif. It is represented ])}' two genera, Diclymictia and Miacis, both 
 from the Eocene of North America. In this family Ave have, as Cope 
 remarks,' "the point of nearest approximation of the Creodonta and 
 
 Carnivora. This is indicated by the fact that 
 the sectorials are sectorials both by position 
 and form, such as are not elsewhere met with 
 in the Creodonta. The genera might readily 
 be taken for members of the Canidse and Vi- 
 verridse (dogs and civets) but for the struc- 
 ture of the astragalus, which is thoroughly 
 creodont." 
 
 The genus Didymictis may be certainly re- 
 garded as the ancestor of the civets, while it is 
 more than probable that Jlincis (Fig. 21.3) was 
 the immediate progenitor of the dogs. In the 
 teeth of Didj/midifs (Fig. 214) the dental formula is not completely 
 
 Fig. 214. 
 
 Fig. 213. 
 
 Fragment of the Lower .Tiiw of a 
 species of Miacis : a, external, fc, 
 internal, and c, vertical views. 
 
 Two Species of Didiimicil^: a, b, c, internal, vertical, and external views of lower jaw of D. dawkin- 
 sianii.t, from ?.i^^ Horn l"ieds ; '/, e, /, D. fim/dsnianus ; d, upper-jaw fragment, vertical view ; e, frag- 
 ment of lett ramus, inner side ; ./', vertical view of same, — all natural size (after Cope). 
 
 I. f, c. 
 
 1 Pm. f , M. f : 
 
 the two true molars tuber-' 
 
 ■' 40. The 
 
 known, but most probably it i 
 
 fourth premolar above is .sectorial in form 
 
 cular. 
 
 In the lower jaw the first true molar has lost much of its typical 
 tuberculo-sectorial structure, which is best seen in the decreased size of 
 the internal tubercle and the tendency of the anterior basal lobe and the 
 
 ■ "The Creodonta,'' American Naturalist, May, 1884, p. 483. 
 
TEETH OF THE VERTEBBATA. 431 
 
 primitive cone to fuse into a cutting blade. A single tubercular molar 
 follows the sectorial, which exhibits, as does the second lower true molar 
 in the dog, a reduced or degraded condition of the tuberculo-sectorial 
 pattern. 
 
 M'lacis has three true molars in the lower jaw, of which the first 
 is sectorial, in this respect resembling very closely the lower jaw of the 
 dog ; its complete dental formula is not known. 
 
 Teeth op the PROSiMiiE. — With this group we enter that division 
 of the Bunotheria which leads out to the monkeys and man. Its palseon- 
 tological history reveals an antiquity quite equal to that of any other 
 of the Monodelphia, continuing backward to the lowest Eocene. It 
 is customary with most naturalists to regard the Prosimice as -widely 
 separated from the Insectivora on account of the higher order of brain- 
 structure which the living representatives of the lemurs display, and 
 they are accordingly placed near the Primates. Owing to the perish- 
 able condition and non-preservation of the soft parts in the extinct 
 forms generally, we will never be able to know the exact structure 
 of their brains, but must be content to judge of its generalized or 
 specialized character by the mould of the cranial cavity, which in 
 many respects is unsatisfactory. 
 
 It can be shown in the ungulate series that the lowest Eocene repre- 
 sentatives possessed brains, judging from the cranial casts, almost as low 
 in the scale of organization as that of the lowest known mammals, and 
 it is likewise true that the brains of the Eocene prosimians A\ere more 
 generalized than those now living. I do not think there can be any 
 radical differences shown to exist between the structure of the brain 
 of such forms as the squirrel shrews (Taupaiadce), the elephant shrews 
 (3Iacroscelidce), and the Galeopithecidce of the insectivores, and the 
 true lemurs (Lemurkke), the fossil Adapis, and others of the Pro- 
 simke. The veiy fact of their remote antiquity and appearance in an 
 age when the brain-development of all the Mammalia was small would 
 of itself lead to the supposition that they too at first possessed brains of 
 lowly organization. It should be here stated that very few skulls of 
 the Eocene prosimians are known. 
 
 The dental formula of the spectrum lemur (Tarsius spectrum) is 
 I. -f-, C. ^, Pm. I, M. |-. Of the two pairs of incisors in the upper jaw, 
 the median is much the larger ; they are closely approximated, long, 
 pointed, and conical, and are surrounded at the base by a prominent 
 cingulum, which is well defined uj^on the anterior face of the crown. 
 The next pair are much smaller, and also have pointed crowns and 
 basal cingula. The upper canines are about equal in size to the 
 median incisors, which they resemble both in the form of their 
 crowns and the cingulum at the base. 
 
 The first premolar is the smallest of the three, and is placed just 
 behind the canine ; its crown is simple and pointed. The next two are 
 larger and imperfectly two-lobed, the internal lobe being represented by 
 a strongly-developed cingulum which continues around upon the outer 
 face of the tooth. The true molars are subequal in size and tritubercu- 
 lar. The two external cusps are well developed, and placed at the 
 external border of the crown. The internal lobe is relatively large, 
 
432 DENTAL ANATOMY. 
 
 and occupies a position opposite tlie interval of the two external. A 
 moderate cingulum is developed on its internal aspect, and continues 
 around to the outside of the tooth. 
 
 In the lower jaw the .single pair of incisors come close together above 
 the symphysis, and completely fill the space between canines ; they have 
 conic crowns and are smaller than the median pair above. The canines 
 are larger than those above, and like them have pointed, slightly re- 
 curved apices. The premolars resemble those above, with the excejDtion 
 that the internal lobe is absent. The true molars are quadritubercular, 
 with the two anterior slightly elevated. A trace of the anterior basal 
 lobe is visible, and is best marked in the first, which brings the struc- 
 ture of the tooth into close correspondence with the tuberculo-sectorial 
 of the Insectivores, and strongly suggests its derivation from it, as so 
 many other examples of a similar kind do. The last molar displays a 
 fifth lobe behind the two posterior ones, and is therefore quintituber- 
 cular. This species is the sole representative of the family Tarsiidce. 
 
 In the typical lemurs of the family Lemuridce the two pairs of upper 
 incisors are separated from each other by a wide space in the centre, both 
 being small and subequal. The superior canines are large ; the pre- 
 molars, with the exception of the first, have a small internal cusp. 
 The molars are quadritubercular by reason of the internal cingulum 
 rising up into a cusp at the postero-external angle of the crown. 
 Various intermediate conditions between the perfect development of 
 this cusp and its almost complete absence are to be seen. The fourth 
 premolar, too, is in some genera like the true molars, in which case the 
 last molar is small. 
 
 The incisors of the lower jaw are two in number upon each side, and 
 are long, slender, laterally compressed teeth, having a procumbent 
 implantation. The canines resemble them very much both in shape 
 and position, being a little larger. The first premolar is large and 
 caniniform, and would be readily taken for the canine at the first 
 glance. The two following are smaller, and usually have simple 
 crowns. The molars are truly quadritubercular, the anterior basal 
 lobe being entirely absent. The last molar may or may not have 
 a fifth posterior tubercle. 
 
 Tketh of the Tillodonta, T^xiodoxta, and Daubentoxi- 
 OIDEA. — The aye-aye ( C 'hiromtjfi) of Madagascar is generally associated 
 with the lemurs in the sub-order Prcmmiw, but naturalists — notably 
 Profs. Cope and Gill — have seen fit to give it a rank equal to that of 
 the lemuroids and j)lace it in a distinct sub-order, Daubentonioidca, on 
 account of the aberrant character of its teeth as compared with the lemurs. 
 There are two other Eocene groups which go with it and constitute the 
 first division of tlie order Bmwtheria, according to Cope. 
 
 The dental formula of the adult aye-aye is, according to Owen, I. \, 
 C. f, P. -f, M. f = 18. The upper incisors are curved as in the Eoden- 
 tia, and deeply implanted in the jaw. Their exposed portions are con- 
 tiguous, their widely-excavated fangs diverging as they proceed back- 
 ward. The incisors of the lower jaw are similar in shape to the upper 
 ones, and are implanted as far back as the coronoid process. They 
 are all covered with enamel, both in front and behind, and grow from 
 
TEETH OF THE VERTEBRATA. 433 
 
 persistent pulps. In the entire investment of enamel they offer an 
 important difference from the incisors of the rodents, which they other- 
 wise closely resemble. The enamel being thicker upon the anterior than 
 upon the posterior face of the tooth causes them to wear into chisel- 
 shaped extremities, whereby the same effective gnawing instruments 
 are produced as in the typical gnawing quadrupeds. The molar and 
 premolar teeth are four in number upon each side in the u^jper, anil 
 three upon eacii side in the lower, jaw. They are implanted after a 
 considerable interval behind the incisors, leaving a wide space or 
 diastema, as in the Bodentia. The first and last molars of the upper 
 series are the smallest, and have single roots ; the second and third 
 larger, and implanted by three fangs each. Their crowns have simple 
 subelliptical grinding surfaces. The molars of the lower jaw are similar, 
 the first being implanted by two roots, the second and third by one each. 
 
 The deciduous or milk dentition is I. f, C. ^, Pm. -|- = 12. In the 
 milk set a small incisor appears upon each side of the median pair, and 
 is not replaced by a permanent one. Two teeth in this set occupy the 
 spaces between the premolars and incisors above, and have been con- 
 sidered canines, they having no permanent successors. The single decid- 
 uous molar in each jaw is succeeded by the permanent premolar. 
 
 The Tillodonta is a group which was discovered and described by 
 Prof. O. C. Marsh from the Upper Eocene deposits of Wyoming Ter- 
 ritory. In the typical genus, Tillotherium, the dental formula, as given 
 by this author is, I. |, C. ^, Pm. f , M. f = 34. The median pair of 
 incisors in each jaw are large and scalpriform, being faced with enamel, 
 as in the rodents. They grew from persistent pulps, as is indicated by 
 the large pulp-cavities at the base. The outer pair are small and did 
 not grow persistently. The canines are much reduced, and placed well 
 back in the alveolar border. The first of the three premolars of the 
 upper jaw is small and simple, the other two being larger and of a more 
 complex pattern. 
 
 The structure of the crowns of the superior molars is not very differ- 
 ent from that of Esthonyx. Two external cusps are present, which are 
 not well separated from each other ; external to them is a broad cingu- 
 lar portion which is produced anteriorly into a process more marked 
 than in Esthonyx. Internally two cusps are present, the posterior being 
 lunate and consisting of a highly-developed cingulum. In the anterior, 
 or that which corresponds with the antero-internal cusp of the quadri- 
 tubercular molar, two well-developed ridges pass outward from its sum- 
 mit, one toward the anterior, and the other toward Ae posterior external 
 angle of the crown, giving it a rounded U-shaped appearance. The 
 pattern of the lower molars is identical -^^ith that of Esthonyx} 
 
 In a general survey of the dentition of this genus I am compelled to 
 dissent from the views expressed by Mr. Tomes, wherein he says that 
 the molar teeth are of the ungulate type, and that the order combines 
 ' characters of the Carnivora, Ungulata, and Eodentia. While it is true 
 that the scalpriform incisors faced with enamel is a condition exhibited 
 by the rodents, a condition also found in the Toxodontia, I fail to 
 
 'See Professor Marsh's monograph of this group, American Journal of Science and 
 Arls,-fo\. xi., ]876, p. 249. 
 Vol. I.— 28 
 
434 DENTAL ANATOMY. 
 
 discover the faintest trace of either carnivorous or ungulate relation- 
 ship. On the otlier , hand, it seems to me that the evidence points 
 strongly to the fact that this group is the direct descendant of Edhonyx, 
 which preceded it in time. This is especially seen in the increased size 
 of the mesial pair of incisors, the reduction of the canines, loss of one 
 premolar in the upper jaw, and the remarkable similarity in the pattern 
 of the molar teeth. That Esthovyx is an insectivore allied to the shrews 
 there is scarcely any doubt. It is also ])robable that this group gave 
 origin to the toxodonts, but the exact connections between them are 
 not now apparent. 
 
 Another family, Stylinodontidw of Prof. ]\Iarsh, makes approaches in 
 this direction in the growth of the molars as well as the incisors from 
 persistent puljjs. 
 
 In the Tceniodontia the incisors are large and scalpriform, and were 
 of persistent growth ; the molar and premolar series are not separated 
 from them by any diastema, in the ]o\vev ja^v at least, and the canines, 
 or those teeth regarded as such, in tlie inferior set also grew from per- 
 sistent pulps, and have grinding crtiwns.^ 
 
 Teeth op the Piumate.s or QuADRU^rAXA. — The teeth of this 
 order are closely affiliated with those of the typical lemuroids in the 
 structure of the molars, and when compared -nith that of the other 
 groups the amount of dental variation is comparatively insignificant. 
 The order is naturally divisible into five families, of which the mar- 
 mosets and platyrrhines of South America and the catarrhines and 
 anthropoids of the Old World, as well as man, constitute the resjjective 
 divisions. Of these families, the marmosets {Hapalida) are the most 
 generalized and approach nearest to the lemurs in several important 
 characters, prominent among whicli are the relatively smooth cerebral 
 hemis])]icres, want of opposability of the thumb and its termination by 
 a distinct claw instead of a nail,' and the jxisscssion of tritubercular 
 instead of quadritulicrcular molars. Since tliey are found only in the 
 New AYorld, and as lemuroids were very abundant in this country in 
 the Eocene Period, it seems probable that they are the derivatives of 
 some member of this group. It is a fact 's\"orthy of notice that in the 
 curious Eocene genus Anajjtomorphus we have a near approach to the 
 anthropoid condition of the teeth. In the sliortness of the jaw and cer- 
 tain cranial jieculiarities it also resembles the higher monlceys. For this 
 reason Cope believes tliat tlie simians have descended directly from this 
 lemur. 
 
 The dental formula of the genus Jfidas is I. ^, C. i, Pm. -|, M. ^ = 
 32, which obtains in the one other living genus. The upper incisors 
 have longitudinally flattened incisive crowns, with a prominent inter- 
 nal ledge at the base. The median pair is the larger, as is gener- 
 
 'Prof. Cope has established this sub-order upon the peculiar condition of the canine 
 teeth of tlie lower jaw, or at least those which he supposes to be such ; the only know- 
 ledge we have of the teeth of the upper ja\v is confined to the large scalpriform incisors. 
 This sub-order must be regarded as provisional until we know more of the upper teeth, 
 as well as the relationship of some genera ajiparently intermediate between it and the 
 TiUodonla. 
 
 ' Many of the lemurs are provided with an opposable thumb, which is terminated by 
 a distinct nail. In this respect the marmosets are even below the lemurs. 
 
TEETH OF THE VERTEBRATA. 435 
 
 ally the case in all the Primates, and are in contact in the median 
 line. The smaller outer incisors follow closely in the dentigerous 
 border of the premaxillaries, after which there is a wide space, almost 
 equal to the width of the two incisors, for the passage of the lower 
 canine. The canines of the upper jaw are comparatively strong for the 
 monkeys, and have pointed, slightly recurved crowns which project far 
 above the level of the other teeth ; there is a deep groove upon their 
 anterior faces. 
 
 The premolars or bicuspids are three in number, and completely fill 
 the interval between the canines and molars. The first is the smallest, 
 and has a prominent pointed external cusp on the grinding surface, to 
 which the cingulum adds a low U-shaped internal portion ; the second 
 and third are similar, except that the internal lobe is no longer cingular, 
 the cingulum furnishing a second internal ledge. The true molars are 
 two in number upon each side, in this respect differing from all known 
 Primates. The only approach to this condition to be met with else- 
 where in the order is in the dentition of man, in whom it appears, as 
 we will hereafter see, that the last molar, or the " wisdom tooth," is 
 gradually becoming rudimentary or defective in the higher races. Vari- 
 ous causes have been assigned in explanation of this fact, one of which 
 is that the greater development of the brain necessitates the expenditure 
 of smaller amount of growth-force upon the maxillary bones, whereby 
 insufficient room is allowed and the tooth stunted, if this be the real 
 cause, it is difficult to understand why in the lowest representatives of 
 the order — and those, too, in which the cerebral hemispheres are pro- 
 portionally the smallest — the complete suppression of the last molar 
 should have occurred. 
 
 The two pairs of lower incisors are small and of the usual incisiform 
 pattern, being considerably smaller than the canines. The lower incisors 
 of the allied genus, Hapale, are proclivous, the canines being relatively 
 small and approximated to them, as in the lemurs, although not to so 
 great an extent. The canines are almost equal to the upper ones in size, 
 and follow the outer incisors without interruption. The three lower 
 premolars are subequal, the summit of the first being elevated above 
 the level of the succeeding teeth. In the first the anterior basal lobe, the 
 principal cone, and an imperfect heel can be indistinctly made out, ^\\n\e 
 in the second and third the internal tubercle is present. In the true 
 molars there are four indistinct cusps; the anterior basal lobe has almost 
 completely disappeared, and all the cusps are of equal height. A care- 
 ful study of unworn teeth will show them to be a still further modifica- 
 tion of the tuberculo-sectorial type, whereby the perfect quadrituber- 
 cular has been produced. 
 
 The next division, Platyrrhines, or flat-nosed monkeys, constitute the 
 family Cebidce, in which the dental formula is I. f , C. ^, Pm. f , ]\I. | 
 = 36. They belong to the continent of South America, and have pre- 
 hensile tails and generally rudimentary thumbs. The canines are usually 
 strong and prominent, and the superior molars have a well-defined ridge 
 connecting the antero-internal with the postero-external cusps, a rem- 
 nant of the tritubercular condition. This ridge is found with varying 
 constancy in the superior molars of all the Primates, and marks the 
 
436 DENTAL ANATOMY. 
 
 connection between the internal cusp and the postero-external tubercle, 
 A\luch generally exists in the tritubercular tooth. The postero-internal 
 cusp, which lies inside and behind this ridge, is the last one which has 
 been added to complete the quadritubercular tooth in the upper jaw. In 
 the s(|uirrel monkeys of this family the lower incisors have a tendency 
 to be proclivous, as in Hapale of the marmosets, thus retaining the 
 lemurine character of these parts. Xo fossil remains of this family are 
 known except from very late geological time, and these do not differ 
 materially from those now living. 
 
 The teeth of the Catarrhincs {Scmnopithecidiv) show a reduction in 
 the number of premolars, whereby the formula I. f, C. \, Pm. -I, M. | 
 = o2, the same as that of man, is reached. The incisors are of the 
 same shape as in man, the central pair being considerably larger than 
 the outer pair. The canines are always strong and powerful teeth, and 
 their apices are always elevated above the other teeth. They reach their 
 maximum of development in the Ijaboons, more especially in the dog- 
 headed baboon, Cynocephalus, in which they are deeply grooved ante- 
 I'iorly. In this group the first premolar below is implanted by jj double 
 fang, with its apex directed upward and backward. The anterior root 
 is naked for some distance, and presents in front a blunt edge which 
 bites against tlie posterior edge of the powerful superior canine, giving 
 to this part of the jaw a peculiar and characteristic appearance. The 
 second lower premolar of Cynocephalus is quadritubercular, with all the 
 cusps well developed, but in the macaques the posterior tubercles are 
 not well defined. Both the upper and lower true molars increase in size 
 from the first to the last, the last lower one being distinctly five-lobed. 
 
 In the semnopithiques the incisors are more nearly equal in size ; the 
 canines are smaller and less deejily grooved than in the baboons ; tlie first 
 and second molars are subequal, Avhile the last lower molar is propor- 
 tionally narrower, but still retains the fifth lobe. The typical cerco- 
 pithiques have the last lower molar quadritubercular and all the molars 
 subequal. Fossil remains of this family are known from the INIiocene 
 and Pliocene deposits of Europe and Asia, but no characters of unusual 
 importance occur in their dentition. 
 
 The next family of this order includes the anthropoid or tailless apes, 
 whicli are also confined to the tropics of the Old World. They consti- 
 tute the family Simiidce, and are distinguished from the preceding fam- 
 ily, Cercopithecidce, i^rincipally by the absence of the tail ; from the suc- 
 ceeding family, Hominidce, by the circumstance that the hallux is oppos- 
 able, whereas in the latter it is in a line with the other digits and is not 
 opposable. Other characters of considerable anatomical importance are 
 also found A\-hich distinguish them from man. 
 
 The teeth of this family are the same in number as those of man, 
 but considerable differences are found in the relative size of the canines 
 and the last molar when compared with that which obtains in the 
 human subject. Although they are organized substantially upon the 
 same plan, the teeth are larger and stronger than in man. The orang 
 {Simia satyrus) is probably the most human-like in its dentition, although 
 in other respects the gorilla and chimpanzee most I'csemble man. The 
 molar teeth in this animal are remarkable for the straight line in which 
 
TEETH OF THE VERTEBBATA. 437 
 
 they are implanted in both jaws, and contrast with the graceful curve 
 they pursue in the normal human mouth. 
 
 The median pair of incisors are larger both above and below ; in the 
 upper jaw they are more than twice the size of the lateral pair, while in 
 the lower jaw they are- more nearly equal. ' Between the lateral pair 
 and the canine above there is a considerable space, into which the lower 
 canine bites. The canines are relatively large, and their apices rise far 
 above the level of the surrounding teeth. They are imperfectly trihe- 
 dral in form, with a trenchant edge behind. These teeth are larger in 
 the male than in the female. 
 
 The premolars or bicuspids differ from those of man in the upper 
 jaw in being implanted by three roots like the molars ; their crowns 
 are very similar to those of man, presenting essentially the same ele- 
 ments. The pattern of the crowns of the molars is like that of the 
 human subject both above and below, but the last molar is as large as 
 the others ; it is implanted by three roots, and is always perfectly formed. 
 In the lower jaw the two posterior molars slightly exceed the first in 
 size, and the last is distinctly five-lobed. The first lower premolar is 
 two-rooited, and has a faint resemblance to the corresponding tooth in 
 the baboons. The second is also implanted by two roots, and its crown 
 agrees with that of man. 
 
 In the other genera minor differences only are to be met with in the 
 form, pattern, and arrangement of these organs. 
 
 The Human Dentition. 
 
 In this connection we come next to consider the teeth of man ; and 
 before so doing I am constrained to make some general remarks in 
 regard to the position he occupies in the zoological scale. While it is 
 undeniable that by virtue of his superior brain-capacity and intellectual 
 development man is to be accorded a place at the head of the animal 
 kingdom, it is nevertheless true that much of his anatomical structure 
 has not been specialized beyond that of many of the lower forms. The 
 fact that different members of the mammalian sub-class have been mod- 
 ified in different directions, some to fit one environment and some 
 another, has led to the specialization of different sets of organs, and 
 that, moreover, in different ways as the surrounding conditions and 
 particular exigencies of the case have required. 
 
 It is these differences which enable the naturalist to construct zoologi- 
 cal definitions of the major or minor groups, such as orders, sub-orders, 
 families, genera, etc. The impracticability of determining which ani- 
 mal is highest or lowest in the scale of organization is thus rendered 
 apparent from the fact that a comparison of different sets of organs is 
 involved. Thus, in their dental, digestive, and limb structure the 
 ungulates surpass all other Mammalia in complexity and specialization, 
 and in these respects may be said to be highest, while in the matter of 
 brain-development they are much inferior to others. The monkey line 
 or Primates, on the other hand, of which man is at the head, retain a 
 comparatively generalized structure of the limbs, teeth, and digestive 
 
438 
 
 DENTAL ANATOMY. 
 
 organs, but have outstripped all others in the development of the cere- 
 bral nervous system. 
 
 It is only upon an evolutionary basis that we are enabled to compre- 
 hend the significance and import of the manifold modifications with 
 which the morphologist is called upon to deal, and it is not at all un- 
 natural that in the consideration of the human or any other dentition 
 the student should first of all bend his energies to the discovery of the 
 relative position which his subject holds in the system. All the evi- 
 dence which anatomical and palseontological science can now bring to 
 bear on the question tends to show that man is the legitimate product 
 and highest expression of the evolutionary forces in that line of devel- 
 opment which began with the Eocene lemuroids, however objectionable 
 this conclusion may be to many. No adequate conception of his place 
 in nature or the structure of any set of his organs can be had without 
 a comparison with the other members of the stem to which he naturally 
 belongs. This reason alone has induced me, somewhat contrar}^ to cus- 
 tom, to give an account of the human dentition in this situation, rather 
 than at the latter part of the present article. 
 
 Looked at from the point of view of the comparative odontologist, 
 these organs present little of general morphological interest beyond that 
 displayed by other Primates ; but from the practical standpoint of the 
 operative dentist they are of the greatest importance. In the course 
 of this account many questions in connection with this latter phase of 
 the subject will doubtless suggest themselves to the reader which are not 
 within the scope of the present part of the work to discuss, its object 
 being merely to outline the anatomy. 
 
 The dental formula of the human subject i 
 Pm. f, M. I = If = 32, the same as that found in the Old "World 
 
 monkeys. Much variation from this num- 
 ber exists, however, by reason of the failure 
 of development of the superior lateral inci- 
 sors and of the third molars, the wisdom 
 teeth or denies sajjientke; these molars may be 
 present in the upper or lower jaw only, or they 
 may fail to develop on one side in one or 
 both jaws, or, again, they may be completely 
 aborted. These vai'iations are most fre- 
 quently met with in the higher races of man- 
 kind, and are said to be of rare occurrence in 
 the inferior races. The teeth are implanted 
 in the alveolar 2)roeess in such a manner in 
 both jaws as to describe a regular parabolic 
 curve, being uninterrupted at any point by 
 the intervention of diastemata or spaces. 
 The summits of the crcnns have, when 
 ajtproximatelv the same level, the canines not 
 
 Fig. 215. 
 
 normally, I. f, 
 
 n 1 
 
 ^- T) 
 
 Superior Maxillary Bone of Man. 
 
 normally developed, 
 
 the apes and mon- 
 more elevated than 
 
 excepted,, thereby affording a marked contrast with 
 keys, in which the crowns of the canines are ahvays 
 the other teeth. 
 
 The incisors are four in number in each jaw, those of the upper being 
 
TEETH OF THE VERTEBRATA. 439 
 
 implanted in the premaxillary bones, which at an early period coalesce 
 with the maxillaries. Of these, the central pair is the larger and has a 
 slightly more anterior position than the lateral ones, on account of the 
 curve of the alveolar border. Their incisive nature is manifested by 
 the possession of a crown, which is bevelled on its palatine or lingual 
 surface ' to a cutting edge, being broader at the extremity than at the 
 
 Fig. 216. 
 
 Inferior Maxillary Bone of Mao. 
 
 base. The adjacent teeth are in contact at their coronal extremities, but 
 on account of the narrower base a slight interval appears between them 
 at the margin of the gum. The root joins the crown without any 
 marked constriction, so that a neck can scarcely be said to exist ; from 
 this point it tapers gradually to an obtuse termination, being imperfectly 
 trihedral in form and slightly recurved. 
 
 In newly-erupted teeth the cutting edge of the crown is divided into 
 three inconspicuous cusps, which soon disappear through wear, leaving 
 it smooth. The basal termination of the crown is indicated by the limit 
 of the enamel covering, which is of greater vertical depth on the labial 
 and palatine or lingual than on the lateral faces, so that if a line be 
 drawn around the tooth at the most extreme basal portion of the 
 enamel, it will touch only the labial and palatine prolongations, and 
 not mark its exact limit on the mesial and distal surfaces. These 
 
 ^ The nomenclature of the various surfaces of a tooth as it stands in position in the 
 jaw, it seems to me, is simplified by employing terms with the following signification : 
 If the tooth-line were straightened out upon each side, the surface which looks away 
 from the condyle would be anterior, and that which is directed toward it would be pox- 
 ierior ; tlie surface directed toward the median line of the mouth would be internal, and 
 that directed away from it external. In this system some confusion may arise with 
 respect to the incisors and canines, in which the anterior surface is internal and con- 
 versely, bwing to the curvature of the tooth-line ; but while it has appeared to me best 
 to speak of the surfaces as if the tooth-line were straight, I have in this paper adopted 
 terms now most familiar to the dental profession, which are represented by the follow- 
 ing: The surface looking toward the anterior part of the mouth and median line is 
 called the mesial surface; its opposite, looking toward the condyle, the distal surface. 
 In the superior row the surface which has been designated the internal I shall term 
 the palatal , and in the inferior row the linqual, while the external surface is the buccal 
 for the molars and bicuspids, and labial for the incisors and cuspids or canines. 
 The triturating surfaces of the molars and bicuspids are termed the masHcaiing surfaces, 
 while the incisive surfaces of the incisors and cuspids or canines are denominated the 
 cutting edges. 
 
440 DENTAL ANATOMY. 
 
 projections of the enamel present convex outlines basally, and are 
 separated from each other by two wide V-shaped notches occupying 
 'the mesial and distal faces. 
 
 The labial aspect of the crown is convex from side to side, as well 
 as from above down\vard, and is of greater vertical than transverse 
 Fig 'ly extent. Upon either side the crown is triangular 
 
 in form, with the apex of the triangle terminating 
 at each free angle of the cutting extremity, and the 
 base directed toward the root ; the basal part of the 
 triangle is interrupted by the V-shaped notch already 
 alluded to. That lateral surface which is directed 
 toward the median line (mesial) is comparatively 
 flat and most produced at the extremity, -while the 
 one which looks away from the median line (distal) 
 A Left Upper Central In- is more roundcd, having its terminal angle less pro- 
 OTiabiaUspect'i^Mnter- duccd. The interior or palatine surface is also tri- 
 nai or Ungual aspect. angular, but the basc is formed by the free cutting 
 edge and the apex turned toward the root. Usually, this surface is 
 nearly flat, but in some, examples it presents a broad central concavity 
 whose depth may be considerably augmented by the presence of two 
 marginal ridges meeting at the radicular extremity or apex of the tri- 
 angle. These ridges, which are homologous with the cingulum of other 
 teeth, sometimes develop a small cusp at their point of junction, in front 
 of which there is usually a deep pit in the enamel — " a favorite site for 
 caries." As a general rule, the cingulum is but faintly marked, and the 
 posterior or palatine face is slightly concave. 
 
 The lateral incisors of the upper jaM- are smaller than the median pair, 
 but have approximately the same form. The labial face is more convex 
 from side to side, and the outer or distal angle of the cutting edge is 
 much more rounded off than in the median. The lingual surface may 
 be slightly concave from above downward, and convex in the opposite 
 direction, without any trace of the cingulum, or, as is most generally 
 the case, it is concave, with the cingulum present, and elevated into a 
 small cusp at the point of junction of the two lateral ridges. The 
 basilar contour of the enamel covering is the same as in the preced- 
 ing tooth. The root is more compressed laterally, of relatively greater 
 „ „„ length, and tapers more gradually to its termi- 
 
 nation, giving to the tooth a more slender and 
 less robust appearance. 
 
 The pulp-cavities of these two teeth have sub- 
 stantially the same shape, and the description of 
 one will answer for that of both. Its form is 
 that of an elongated tube, gradually increasing 
 in diameter from the apical foramen in the apex 
 A Lower incifior of Man: a, of the root to a point whicli nearly coincides 
 
 anterior, and (,, lateral view. ^^.j^,^ ^j^^ summit of the V-shapcd UOtch iu the 
 
 enamel on the lateral surface of the crown, where it becomes contracted 
 in an antero-posterior direction, but enlarged in its transverse diameter. 
 It is prolonged upon either side into a slight cornua, which reaches but a 
 short distance beyond the level of the general cavity ; the one which cor- 
 
TEETH OP THE VEBTEBBATA. 441 
 
 responds to the internal or mesial angle of the cutting edge of the crown 
 is a little the longer of the two. 
 
 The two pairs of lower incisors reverse the condition of the superior 
 set, in that the central ones are the smallest. Their crowns have sub- 
 stantially the same pattern as those in the upper jaw, with the exception 
 that an internal or lingual cingulum is never developed. They are readily 
 distinguished from those above by their smaller size and greater lateral 
 flattening of the roots. The pulp-cavity and basal enamel contour are 
 like the corresponding teeth above. 
 
 The cuspids, canines, or " eye teeth," are the next in order behind the 
 incisors ; in both jaws they completely fill the gajj between these latter 
 teeth and the bicuspids, being in contact with yig 219. 
 
 them at the mesial and distal extremity of the 
 crown. They are in every way stronger and 
 more robust than the incisors, and are im- 
 planted by roots whose length, proportionate 
 to that of the crown, is much greater. In the 
 upper jaw these are indicated on the external 
 surface of the maxillary bone by a vertical 
 ridge or swelling which in many skulls extends 
 quite as high as the lower border of the ante- 
 
 • ^oyrio A Left Superior Human Canine: 
 
 Iior nares. a, external, and 6, iuternal 
 
 The crown is terminated by an obtuse point, ^'^^■ 
 which has a position in a line with the longitudinal axis of the root. 
 Upon either side of this cusp the terminal extremity slopes away, but 
 still retains a blunt cutting edge. When the median cusp is reduced by 
 wear the crown does not look very much unlike that of an incisor ; its 
 labial or external face is broader above than below,' and convex in both a 
 transverse and a longitudinal direction, as in the incisors ; the palatal or 
 internal surface is also bevelled, and the lateral surfaces (mesial and 
 distal), or those which lie adjacent to the contiguous teeth, are likewise 
 somewhat triangular in form, but more rounded. In the superior 
 canines a slight ridge descends upon the external or labial face from 
 the summit of the terminal cusp to the neck, but is absent in the 
 corresponding teeth below. 
 
 The internal or palatine aspect is slightly convex from side to side, 
 but concave from above downward. The palatine convexity is occa- 
 sioned by a well-marked vertical ridge which extends from the summit 
 of the terminal cusp to the cingulum below ; this latter structure is 
 usually well defined, being stronger in the upper than in the lower teeth. 
 There is, as a general rule, a prominent basal cusp at the junction of 
 the two lateral ridges which connects with the vertical ridge, leaving a 
 deep pit upon either side — a spot where caries very frequently occurs. 
 
 As already stated, the extremity of the crown slopes away upon either 
 
 ' When the terms above, helow, superior, and inferior are used in connection with a 
 single tooth, they refer to the free as opposed to the implanted extremities : in the 
 upper jaw the part of the crown which is really above is that which joins the root, but 
 in the lower jaw it is the reverse of this. It is convenient to use these terms for all 
 teeth, as they correctly apply to the lower teeth, so that when we speak of the superior 
 extremity of the crown, the free or terminal part is meant, whether it belong to the 
 upper or lower jaw. 
 
442 DENTAL ANATOMY. 
 
 side of the median cusp ; that side which lies next to the premolars or 
 bicuspids is longer than that which is directed toward the incisors, so 
 that the distal or posterior moiety is greater than the mesial or anterior. 
 This inequality of the two sides exists in both pairs of the canines, being 
 less marked in the lower than in the upper ; it furnishes a very useful 
 rule by which a canine can be referred without difficulty to its proper 
 side of the mouth. 
 
 The inferior cuspids or canines differ principally from those above in 
 the shorter root, blunter median cusp, and less-marked posterior or lin- 
 gual cingulum and basal cusp. The roots of both are thicker labially 
 than lingually, and are generally traversed by a vertical groove upon 
 either side. 
 
 The bicuspids or premolars are four in number in each jaw, and afford 
 a further complication of the pattern of the crown by reason of the ele- 
 vation of the basal cingulum into a strong in- 
 FiG. 220. ternal cusp. In proportion as this part of the 
 
 crown is well marked and complicated, there is 
 a corresponding disposition to increase in the 
 0- (f SL^ / /f iIB'^ number of roots or fangs. These teeth, as their 
 
 name implies, are provided with two cusjis to the 
 cro\'\-n ; those of the superior set are of subequal 
 dimensions and considerably exceed the lower 
 
 First I pper Bicuspid or Pre- . . rni ^ ■ i i • • i i 
 
 molar of Man: a, vertical ones in sizc. ihe crowu ot the bicuspid, when 
 view of the crown;!,, lateral ^.-^^g^ vertically, presents an imperfectly quad- 
 rate outline, which is most distinct in the second, 
 and are broader than long. Two strong cusps, of which one is exter- 
 nal and the other internal, occupy the grinding face, and are separated 
 'oy a deep notch or valley, deepest in the centre. The anterior and pos- 
 terior margins of this valley are bordered by slight ridges which con- 
 nect the anterior and posterior extremities of the cusps ; the anterior of 
 these is a little moi'e elevated than the posterior, and forms a useful 
 guide in determining the mesial and distal surfaces of the tooth, and 
 consequently the side of the jaw to -which it belongs. In some instances 
 the enamel forming the floor of the valley and adjacent sides of the 
 cnsj)S and ridges is quite smooth, but most frequently it is considerably 
 wrinkled and thrt )-\\n into a number of minor cusps and ridges, with 
 intermediate indentations which offer receptacles for the lodgment of 
 food. 
 
 Of the two cusps, the external is slightly the larger and more ele- 
 vated ; it likewise has a greater antero-i:)osterior extent. Its form is 
 very much like the entire cru^^'ii of the cuspid, terminating superiorly 
 in a median cusp, from which the cutting edge gradually slopes away 
 upon either side. The internal vertical rib is also present, but the 
 external is absent. The internal or palatine cusp is thicker transverselv 
 than the buccal, and is more rounded. On account of the connecting 
 ridges it has somewhat of a crescentic pattern. 
 
 Commonly, there is, to all apiDcaranee, but a single root, which is 
 traversed upon the mesial and distal faces by vertical grooves which 
 may unite near the apex, causing it to become divided. These vertical 
 grooves are the external indication of t\\o pulp-cavities in the implanted 
 
TEETH OF THE VEBTEBBATA. 
 
 443 
 
 Second Lower Human Bicus- 
 pid : 0, b, vertical and lat- 
 eral views. 
 
 extremity, which unite about midway of the root, and are thence con- 
 tinued upward into the crown as a common cavity. The cavity thus 
 formed is of greater transverse than antero-posterior' extent; in the 
 vicinity of the neclv it is little more than a narrow transverse fissure, 
 which widens somewhat above, and is prolonged into two cornua corre- 
 sponding to the two cusps. The external of these is the larger and most 
 elevated. 
 
 While this condition of the roots and pulp-cavity is the one usually 
 to be met with, nevertheless two roots are frequently found in the 
 first bicuspid, and three roots are occasionally 
 developed, two of which support the outer cusp ; Fk*. 2'J 
 
 the pulp-cavity has then, of course, three divis- 
 ions. 
 
 The principal differences between the upper 
 and lower bicuspids or premolars are seen in the 
 size of the internal cusp as compared with the 
 external, the more cylindrical form of the root, 
 and the almost complete absence of the vertical 
 grooves, on account of which the pulp-cavity 
 is, as a general rule, single. The crown con- 
 sists of a large, somewhat conical external cusp, very convex without, 
 to which is added a low lunate internal cingular ridge. The internal 
 vertical ridge of the external cusp joins this cingulum near its central 
 portion, leaving a deep pit upon either side where the destructive agen- 
 cies of decay on the crowns of these teeth exhibit themselves most fre- 
 quently. The degree to which this vertical rib is developed is subjected 
 to great variation ; it may be almost entirely absent in some individuals 
 or strongly developed in others. The crown of the second or posterior 
 bicuspid or premolar is more quadrate in outline than the anterior or 
 first ; the internal cusp is better developed, and frequently shows a tend- 
 ency to form two. 
 
 The normal number of true molars is twelve, three on either side in 
 each jaw, but, as already remarked, the last in both series may be absent. 
 In a series of adult skulls of various civilized races which I have 
 examined, twelve out of forty had one or both of these teeth wanting 
 from the upper series, and in the lower jaw^ 
 the proportion was ten to thirty. It is highly 
 probable that in many of these cases these 
 teeth had been present, but had disappeared 
 early in life. Many examples could be cited 
 in which the last or third molars wholly fail 
 to be erupted, and it is established upon good 
 authority that in many families one, two, or 
 all of these teeth are habitually absent from 
 generation to generation. 
 
 In the lower jaw the three molars in the 
 more typical lower races are equal in size and substantially alike in pat- 
 tern ; their crowns are quadrangular in section, with the angles consider- 
 
 ' By antero-poHerior in this connection is meant the diameter which corresponds with 
 the long axis of the jaw. 
 
 Fig. 222. 
 
 First Lower Human Molar : o, verti- 
 cal view of the crown ; 1. anterior; 
 2, posterior aspect; 6, side view. 
 
444 DENTAL ANATOMY. 
 
 ably rounded off. They support four principal cusps, as in the quadri- 
 tubercular molar generally, together with a fifth one behind, which is 
 strictly homologous with the heel of these teeth in the more generalized 
 members of the Primate section. These are separated by four distinct 
 fissures arranged in the form of a cross ; \vhere the two limbs cr(jss each 
 other they widen out into a median valley deepest in the centre. The 
 longitudinal of these, or the one which separates the external from the 
 internal principal cusps, terminates in a posterior bifurcation A\hich con- 
 stricts off the fifth cusj) or heel. The enamel lining this valley is, in 
 perfectly unworn teeth, much corrugated, so that it is sometimes difficult 
 to distinguish the principal cusps. 
 
 They are, with the excej)tion of the last or third molar, implanted by 
 two antero-posteriorly flattened roots, Mhich join tlie crown at the mod- 
 erately \\ell-defined neck. These may be connate, having the two roots 
 indicated only I)y a vertical groove upon either side. Each root is hol- 
 lowed out in the centre to receive the radicular portion of the pulp, the 
 cavity corresponding with the external form of the root. These unite 
 into a common cavity above, which at about the time the tooth is erupted 
 is relatively very large, but which becomes smaller with age, and is final- 
 ly in old age obliterated through progressive calcification. The body 
 of the cavity is terminated superiorly by cornua corresponding to the 
 five cusps of the crown ; of these the two anterior are most prolonged, 
 and reach slightly above the inferior limit of the outer enamel covering. 
 
 In the higher races the last or third molar is usually smaller than the 
 first and second, and does not have the cusp so well defined ; but in 
 many of the negro skulls I have examined it is nearly as large, and 
 quite as well formed, as the two anterior to it. This tooth is more 
 constant, both as regards presence and form, than the corresponding 
 tooth above. It is, as a rule, two-rooted, but these rcHjts may be 
 confluent, in which case two vertical grooves mark a tendency in this 
 direction. 
 
 The superior molars, like those in the lower jaw, are three in num- 
 ber, and have quadritubercular crowais normally, but many examjjles 
 can be found in which the postero-internal cusp, the last one added in 
 the quadritubercular molar, is little more than a cingulum,^ and is 
 scarcely entitled to the appellation of a cusp. In such cases it fre- 
 quently has a position internal to the antero-internal cusp, and all 
 stages between that and its normal position are to be met with. 
 
 The gi'inding face of the crown is of a squarish form, bearing a 
 
 ' It is probable that this condition, of which 1 have seen a number of examples in 
 the higher races, is a degenerate one, and is an effort to return to the tritubercular 
 stage. Dr. Harrison Allen, in a communication to the Academy of Natural JSciences 
 of Philadelphia, has recently called attention to the fact that in senile changes those 
 structures which have been added last in the coui-se of evolutionary growth are the 
 first to disappear. Although this condition cannot be said to be in any way depend- 
 ent upon individual senility, it is in all probability the result of senility of the race, 
 wherein retrogressive modifications of any set of organs are first apjiarent in those 
 parts which were the last to appear. It should be stated here that to Dr. Allen is due 
 the credit of having prepared the way for all the more important generalizations that 
 have been made in regai-d to the evolution of the quadritubercular tootli from the m(5re 
 primitive types. Pie demonstrated that the postero-internal cusp of the human molar 
 is an outgrowth from the cingulum. 
 
TEETH OF THE VEBTEBRATA. 445 
 
 cusp at each angle. Of the two external, the anterior is slightly the 
 
 larger, and is usually connected with the antero-internal by a strong 
 
 ridge which skirts the anterior margins of the crown. The posterior 
 
 is separated from it by a fissure which terminates internally in the 
 
 median valley ; it is also connected with the antero-internal cusp by a 
 
 ridge, the oblique ridge. From its posterior 
 
 margin a well-developed cingulum passes in- '^^^' ~"'^- 
 
 ward on the posterior border of the crown to 
 
 join the postero-internal cusp, of which, as 
 
 already remarked, this latter is a part. 
 
 The antero-internal cusp is the largest of 
 the four, and by reason of its union with 
 the cross-ridges above mentioned has a some- 
 what crescentic appearance. It is placed at ^'"''erTirandM^rUcai^vrew"''''*" 
 the apex of a V which opens externally and 
 
 encloses the median valley. The postero-internal cusp in the specimen 
 figured stands a little posterior and internal to the last mentioned, being 
 separated from it by a deep groove ; it is little more than an enlargement 
 of the strong posterior cingulum. 
 
 The roots are three in number, of which two are external or buccal, 
 and support the two outer cusps, and one internal or lingual, supporting 
 the two internal cusps. The two outer are not unfrequently connate, 
 in which case the line of separation of the radicular portions of the 
 pulp-cavities is indicated by a vertical groove. The palatine is the 
 largest and longest root of the three. 
 
 While the structure here described usually obtains in the first and 
 second molars, the last is more simple and variable. In the more civil- 
 ized races it is exceptional for these teeth to be regular either in form or 
 position, so great is their variability. Tlie crown resembles in a gen- 
 eral way those of the first and second molars, except that the oblique 
 ridge is generally absent and the two internal cusps are blended together. 
 The roots are connate and somewhat curved at their implanted extrem- 
 ity, and the pulp-cavity is single. 
 
 Oedusion of the Teeth. — The diagram (Fig. 224) on p. 446 represents 
 the occlusion of the teeth. It has been previously stated that in a well- 
 formed denture no one tooth rises higher than its fellows ; that is, if 
 the crowns of the teeth in position be turned, cusps and cutting edges, 
 upon a plain or even surface, each tooth rests upon this surface. From 
 this arrangement there is nothing to interfere with a perfect occlusion. 
 Still, the fact must be recognized that while the above-described arrange- 
 ment is true of a perfectly-developed jaw and teeth, yet so rarely is it 
 found that it may be considered an ideal denture. 
 
 It has also been stated that the superior arch or row of teeth describes 
 the segment of a larger circle than does the inferior row ; this being the 
 case, when the two are brought in contact, as in normally closing the 
 mouth, the anterior superior teeth are thrown slightly over and anterior 
 to the corresponding inferior teeth. Also with the bicuspids and molars, 
 the external cusps of the superior on6s are in closing slightly external to 
 the corresponding cusps of the inferior. Another serviceable peculiarity 
 is the noticeable absence of an exact opposition of tooth to tooth in clos- 
 
446 DENTAL ANATOMY. 
 
 ing, as will be seen by the diagram : the greater width of the superior 
 central covers the width of the inferior central and a small portion of 
 the inferior lateral ; this brings the superior lateral over the remainder 
 of the inferior one and the mesial fourth of the inferior cuspid, while 
 the cusp of the superior cuspid fits into the concave space between the 
 cusp of the inferior cuspid and the first bicuspid. In like manner, this 
 
 Fig. 224. 
 
 irregularity of opposition is maintained in all the teeth, so as to give 
 each tooth a bearing on two teeth, except the superior third molar, 
 which has but the corresponding tooth in the lower jaw for an antag- 
 onizer. This irregularity of opposition contributes to the efficiency of 
 the teeth in mastication, and is a valuable feature when a tooth is lost 
 from the arch in either jaw, for by this arrangement the tooth in partial 
 antagonism with the one lost still maintains a portion of its usefulness 
 by its occlusion with yet another tooth. 
 
 The Deciduous or Temporary Teeth (Fig. 225), twenty in number, are 
 
 smaller than the permanent set, though they resemble them in their gen- 
 eral conformation of crown and root, the bicuspids of the permanent set 
 
TEETH OP THE VERTEBBATA. 
 
 447 
 
 not being represented in the deciduous dentine. The formula, when they 
 are normally developed, is I. f, C. \, M. |-=^ = 20, the premolars or 
 bicuspids being confined to the permanent set. A marked point of dis- 
 similarity, as compared with their successors, is in the termination of the 
 enamel on the neck of the tooth. In the permanent teeth the gradual 
 completion of the enamel on the border of the cement marks but indis- 
 tinctly the point of union of these two structures, while at the base of 
 the deciduous crown the terminating enamel on the buccal and labial 
 surfaces is recognized by a ridge or well-defined border which unmis- 
 takably marks its limitation and develops a well-constricted neck. This 
 difference is often of importance in deciding as to whether a tooth in 
 question belongs to the deciduous or permanent series. These teeth, 
 from the fact that their crowns are largely calcified before birth, are 
 much less liable to vices in conformation than their successors, but from 
 deficient nutrition, want of use, and neglect not unfrequently become an 
 easy prey to the ravages of dental caries. In common with a large class 
 of the order to which man is closely allied, the difference in ntunber 
 between the deciduous and permanent set is twelve, the additional 
 teeth being without predecessors. 
 
 The accompanying figure (226) represents the denture of a child about 
 
 Fig. 226. 
 
 seven years of age. Twenty deciduous teeth, ten in each jaw, and the 
 four first permanent molars, are erupted. The second permanent molar 
 is seen in the crypt in the posterior part of each maxilla. Commencing 
 with the median line, to the right of it and just above the erupted decid- 
 uous central incisor, we observe the permanent central with its crown 
 fully calcified and the root partially so. In this case the crown of the 
 permanent tooth stands in front of or on the labial side of the partially 
 absorbed root of the deciduous central. This is not its constant relative 
 position ; not unfrequently the deciduous root is in front of the crown, 
 as is seen in the adjoining lateral incisor. In this case the crown of 
 
448 DENTAL ANATOMY. 
 
 the permanent lateral has the position which it invariably maintains. 
 The crown of the jjermanent cusjjid is here normally located quite above 
 the root of its predecessor, and at the side of and in close proximity to 
 the wing of the external nares. Xext in position, a little below and 
 slightly posterior to this cuspid crown, is that of the first bicuspid, this 
 and its tellow, the second bicuspid, are located between the roots of 
 their respective predecessors, the first and second deciduous molars. 
 The same is true of those on the other side of the jaw, and, with slight 
 variation, the same relative positions of the deciduous roots and perma- 
 nent crowns are observed in the inferior maxilla. It is above stated 
 that the figure represents the teeth of a child about seven years of age. 
 The first permanent molars, it should be noted, are at this age erujjted 
 and in position, though their roots are not quite completed. The per- 
 manent central incisors will be the next to take their position at about 
 the age of eight, followed l)y the laterals at nine, the first bicuspids at 
 ten, the second bicuspids at eleven, the cuspids from twelve to thirteen, 
 and the second molars from twelve to fourteen, A\-hich completes the 
 eruption of the permanent teeth, with the exception of the third molars 
 or -wisdom teeth, these may take their position at eighteen or some years 
 later. The anatomy of human dentition is further illustrated in plates 
 placed at the end of this paper (see p. 505). 
 
 Teeth op the Oaenivoea. 
 
 Our knowledge of the philogenetic history of the unguiculate series 
 lias so increased within the last fe-w years that it is now a matter of great 
 difficulty to say just M'hat forms should be included in the order Curnh- 
 oiri, as at present defined. If ^^'e take into account the living forms only, 
 no one will hesitate in fixing its limit and giving to it a moderately good 
 definition ; but when the fossil representatives are considered, the interval 
 between it and some of the contiguous orders, especially the lasediwra, 
 is brought down to extremely small limits. AA'e have already seen that 
 the JTiarifJtr ajjproach the dogs and civets in the Oirnivora on the one 
 hand, and the Leptk-ttdiv of the Innrctivora on the other. If a dog, 
 bear, cat, and seal, all of which are admitted to belong to the Cdndrora, 
 I)e selected, and a careful comparison of their anatomical structure insti- 
 tuted, the differences bet\veen them ^viIl be found to be much greater 
 than between such forms as Stypolophun, Ccntetes, Mineix, and the dogs 
 and civets. 
 
 Every increment to our knowledge of the more exact relationship 
 of the various groups seems to bring us nearer to the conclusion that our 
 present classification is largely a matter of convenience, and often fails 
 utterly to express the deeper and more important facts of origin and 
 ancestry. Such reflections bring us abreast ()f the question. What is an 
 order, a family, or a genus, etc. ? And just here we approach a prob- 
 lem as to the solution of which no two naturalists agree. 
 
 It apjx'ars to me that the only way out of these difficulties is to con- 
 sider the test of ancestry the only true basis of affinity. If it can be 
 shown, for example, that any given assemblage of organic forms have 
 descended from a common ancestor, however much they may differ 
 
TEETH OF THE VERTEBRATA. 449 
 
 among themselves, such a line or branch constitutes a natural division. 
 Viewed from this standpoint, there can be little doubt that the order 
 Carnivora represents the terminal extremities of several distinct branches, 
 which arose not from one, but from two or perhaps three points in the 
 Insectivora. The same reasoning holds good for many other orders. 
 
 The order Carnivora, as at present understood, is divisible into two 
 sub-orders — Fissipedia, or the land carnivores, and the Pinnipedia, or 
 aquatic flesh-eaters. The latter division includes the seals, sea-lions, 
 and walruses, and is distinguished by the flipper-like modification of 
 the feet for progression in the water, as well as by several important 
 cranial characters. They are all known to be diphyodont, but the milk 
 teeth disappear early ; in some cases this occurs before birth, and in 
 others a few weeks after. The teeth always possess comparatively 
 simple crowns, which are either simple cones, as in the majority of 
 the Cdacm, or laterally compressed, like the premolars of the dog, 
 with smaller cusps along the edge, giving a well-defined serrated 
 structure. 
 
 There are three families of this group, viz. the Phocidce or seals, the 
 Otaridce or sea-lions and sea-bears, and the Triohecidce, or walruses. 
 In the common seal {Phoca vitulina), which is a good example of the 
 first, the dental formula is I. f , C. ^, Pm. |-, M. -|- = 34. The central 
 pair of incisors above (Fig. 227) are the smallest, with sharp-pointed, 
 
 Fig. 227. 
 
 Vertical View of the Upper Jaw of a Harbor Seal {Pfioca vitulina). 
 
 slightly hooked crowns ; the next are similar in shape, but a little 
 larger, while the outer pair are abruptly increased in size. These are 
 separated from the canine by a diastema to admit the lower canine. 
 The canine is a powerful tooth, with a conical recurved crown, and is 
 deeply implanted in the maxillary bone. In the specimen figured, which 
 is a young individual, it is remarkable for the veiy large size of the 
 pulp-cavity, which extends nearly to the apex of the crown. The first 
 premolar follows just inside and behind the canine, giving a crowded 
 appearance to the first two premolars, the longitudinal axes of which 
 are directed very obliquely to that of the succeeding teeth : it has no 
 deciduous predecessor, as the corresponding tooth in the dog, and is one 
 of the many examples in which it is difficult to say whether it should 
 be relegated to the milk dentition as a persistent milk molar or whether 
 it should be referred to the permanent set. It is implanted by a single 
 root, also remarkable for the size of the pulp-cavity, and has a crown 
 
 Vol. I.— 29 
 
450 DENTAL ANATOMY. 
 
 with a principal hook-shaped cusp, a small posterior basal cusp, and a 
 strong internal cingulum. 
 
 The next three premolars are similar, except that they are larger, 
 implanted by two roots, and have two posterior accessory cusps, the 
 hindermost of which is very small. The single molar differs from the 
 rest of the teeth in advance of it in having an anterior basal cusp, being 
 relatively thicker at the base of the crown, and Avith a moderately well- 
 defined internal cingulum, which displays a tendency to develop inter- 
 nal cusps. 
 
 The incisors and canines of the lower jaw are like those above, but 
 the two incisors of each side are separated at the median line. The first 
 p , ,t,„ premolar is single-rooted and 
 
 somewhat larger than its fellow 
 aljove. The crown displays a 
 median cone with three poste- 
 rior accessory cusps, and one 
 very minute anterior one. The 
 following teeth, including the 
 molar, are all similarly con- 
 structed, but have the anterior 
 
 Vertical \ie\v of the Lower Jaw of a Harbor Seal. i i i , , t n ^ 
 
 basal cusp better defined. 
 
 In the hooded seals (Cystophora) the incisors are two upon each side 
 above, and one upon each side below. The canines are comparatively 
 large and powerful, while the molars and premolars are small and reduced 
 to simple conical bodies, similar to the teeth of the cetaceans. In another 
 genus iStenorhynchus) the teeth are remarkable for the great length of 
 the cusps, and in one species, Lejytonyx, for the curvature of the acces- 
 sory cusps toward the principal one, thereby resembling the trident 
 of a fishing-spear. 
 
 A good example of the dentition of the Oinrida: is furnished by the 
 fur seal (^Callorhynvs ursinus), which can usually be found in museums. 
 The dental formula is I. |, C. {, Pm. |, M. f = 36. The two median 
 pairs of incisors above are subequal and laterally compressed. They 
 each present a deep transverse notch in the summit of the crown, into 
 which the ineisiform extremities of the lower incisors bite ; the outer 
 pair are larger and sharp-pointed. The canines are relatively longer 
 and more slender than in the seals, and liave a well-defined posterior 
 trenchant edge. The succeeding teeth are all alike in form and size, 
 being implanted by single fangs. Their crowns are of a triangular 
 shape wlien viewed from the side, and present a single cusp. It fre- 
 quently happens that the bases of these teeth just where they emerge 
 from the gums are very much eroded, the cause of which is not at 
 present well understood. 
 
 Both the Phocidcp and Otaridce are remarkable for their comparatively 
 weak and slender jaws, the backward direction of the coronoid process, 
 and the great distance intervening between its base and the last tooth. 
 In the seals the palate is very broad posteriorly, and the last tooth does 
 not extend behind the anterior root of the zygoma, whereas in the sea- 
 lions the palate is long and narrow, and the last tooth is placed consid- 
 erably behind the anterior termination of the zygomatic arch. 
 
TEETH OF THE VERTEBRATA. 451 
 
 The TrichechidcB or walruses exhibit tlie most anomalous condition 
 of the dental organs of any pinniped carnivore so far known, in that 
 two enormous tusks are developed in the upi)er jaw, which occupy the 
 position and fulfil the functions of canines. Owing to the transitory 
 character of some of the other teeth, it is difficult to assign a definite 
 dental formula to this animal. Prof Flower makes it out to be I. -J-, 
 C. \, Pm. I, M. ^. Besides these there are, according to Tomes, sev- 
 eral other small teeth to be found frequently in the position of the inci- 
 sors, and he is disposed to regard them as the rudimentary representatives 
 of the permanent normal ones in other animals ; there can be little doubt 
 that he is correct. Rudiments of true molars are also not unfrequently 
 present in the back pai-t of the jaws. The incisors and molars are small 
 and simple, and are soon worn down even with the gums into obtuse oval 
 grinding surfaces. The canines of the upper jaw protrude far below the 
 level of the symphysis, and grow from persistent pulps. They are com- 
 posed of dentine with a thin investment of cementum. Tomes says of 
 them : " These great tusks are employed to tear up marine plants and 
 turn over obstacles, the walrus feeding upon Crustacea and also upon sea- 
 weed, etc. ; they are also used to assist the animal in clambering over 
 the ice ; as they are of almost equal size in the female, they cannot be 
 regarded as weapons of sexual offence, but they are undoubtedly used 
 in the combats of the males." 
 
 The walruses and sea-lions agree with respect to the use of the hind 
 limbs for progression on land, being able to walk on all fours fairly 
 well ; in the seals, on the other hand, the posterior members are rotated 
 backward, and permanently fixed in this position, so as to be of little 
 or no use in walking. In this respect they approach nearer to the 
 cetacean condition. 
 
 Viewing the Pinnipedia as a whole, I am inclined to think that the 
 relationship existing between them and the Fissipcdia is more apparent 
 than real ; and although palaeontology does not at present permit us to 
 judge, I am of the opinion that they will ultimately be found to have 
 been derived from an entirely diflFerent ancestry. Fossil remains are 
 known as far back as the Miocene, but all that have so far been found 
 are typically pinniped. The simple structure of the teeth finds a par- 
 allel in the Insectwora in the teeth of the lower jaw of the genus 
 Mesonyx, already described, which Cope believes to have been more or 
 less aquatic from the evidence afforded by some of the limb bones. This 
 genus or an allied one may have been the progenitor of the pinnipeds, 
 but too little is known of the skull-structure to say anything about the 
 affinities between them. 
 
 The fissiped Carnivora are more extensive, both in number and 
 variety, than the pinnipeds, and enjoy a wider range of distribution. 
 Some of them are almost exclusively aquatic in habit, while others are 
 arboreal, fossorial, or terrestrial. It is in this group that we meet with 
 the highest specialization of the dental organs for the purpose of seiz- 
 ing, lacerating, and devouring living prey. In many the claws are 
 extremely sharp and hook-shaped, and are provided with a special 
 apparatus by which they are made retractile, thereby rendering them 
 efficient organs of destruction and prehension as well. The feet are 
 
452 
 
 DENTAL ANATOMY. 
 
 Fig. 229. 
 
 not modified into flippers, as in the pinnipeds, but constitute distinct 
 " paws," A\hich in the aquatic forms have webbed toes. The canines 
 are always present and generally of formidable j)roportions, while the 
 sectorial or shearing apparatus is present only in those that subsist 
 exclusively on an animal diet. 
 
 They have been divided Ijv Prof. Flower into three groups, which he 
 has called the Cijnaidea, Ailtiroldeu, and Arctoidea, defining them by the 
 characters of the otic bulla and the base of the skull. The first of 
 these includes the dogs, wolves, and jackals, etc., and in all probability 
 represents the central group. From it the civets, cats, etc., consti- 
 tuting the Ailuroidea, branch ofP on the one hand, while the bears, 
 weasels, raccoons, etc. are closely connected on the other. The Oynoidea 
 comprises two families — according to most authors only one ; these are 
 the Canidcc, or <logs, \\-olves, foxes, etc., and the Jlegalotidce, including 
 the single genius Mcgalotis, or the fennec of Africa, A\hich, for reasons 
 which ^\'ill appear hereafter, I am strongly disposed to regard as an 
 entirely distinct family. 
 
 A typical dentition of the Canidce has already been described in that 
 of the dog. About the only dental variations of importance to be seen 
 
 in this family consists in the re- 
 duction of the number of premo- 
 lars, addition or subtraction to the 
 number of upper true molars to or 
 from that of the dog, subtraction 
 from the lower molar series, and 
 slight modification in form (jf the 
 sectorials. Upon these variations 
 jjrincipally some thirteen or fifteen 
 genera have been defined. The 
 dental formula for many of the 
 genera is the same as that of the 
 dog, I. I, C.j, Pm.f, M.| = 
 42, but the extinct ]\Iiocene genus 
 Amphkyon (Fig. 229), found both 
 in this country and Europe, had 
 three true molars in the upper jaw. 
 In another extinct genus {Enhy- 
 d roc yon), described by Cope from 
 the iMiocene of the John Day beds 
 of Oregon, the premolars are re- 
 duced t(i three in each jaw. OHgo- 
 bimus is the name given by this 
 author to another extinct genus 
 from the same locality, in which 
 The principal part of the skull is 
 represented in Fig. 230. Still another genus of this family has been 
 described l)y the same author from the rich fossiliferous deposits of that 
 region under the name of Hywiiocyoii, Avhich has three premolars above 
 and below, with only .a single true molar above. 
 
 The sectorials of the more typical Canida: are like those described in 
 
 Skull of Am,pfiicyon cuRpigpriia, Tojie, ^-ith last 
 superior molar lost, one-half nutural size, from 
 the John Day beds of Oregon, (after lope). 
 
 the molar formula is Pm. 4, jM- \- 
 
TEETH OF THE VERTEBRATA. 
 
 4.-j3 
 
 the dog, but in some genera — notably Temnocyon of Cope— the heel of 
 the lower sectorial, instead of being basin-shaped, retains the more primi- 
 tive structure, and consists of a single trenchant cusp (see Fig. 231). 
 
 In the extinct genus Ailurodon of Leidy the dental formula is the 
 same as in the dog, but it approaches tlie cats, and especially the hytenas. 
 
 Fig. 230. 
 
 Fig: 231. 
 
 Portion of Skull of Oligobunus erassivuUii.^, Cope, one-half natural size : la, right maxillary bone from 
 below ; 2, right mandibular ramus from above (after Cope). 
 
 in having three cusps to the blade of the superior sectorial, whereas the dog 
 has only two. The premolars too are more robust than in the dog, consti- 
 tuting another approach to the 
 condition of the Hycenidce. The 
 skull is represented in Fig. 232. 
 The genus Ichtithermm of Gau- 
 dry (Fig. 233), from the Miocene 
 of Pikermi, Greece, is an allied 
 genus, but the third molar of the 
 lower jaw is absent, leaving a for- 
 mula, 1. 1, C. I, Pm. f , M. I = 40. 
 In one species (/. robustum) the 
 last superior molars have nearly 
 the same proportions as in the 
 dog, while in another (J. hippa- 
 rionurn) the last molar is consid- 
 erably reduced in size. It will 
 thus be seen that these two genera 
 depart from the central or typical 
 Canida}, and establish close con- 
 nections withthe Hycenidce, which 
 are closely affiliated with the cats 
 and belong to the Ailuroidea. 
 Cope has suggested that Ailu- 
 rodon is the ancestor of the hysenas ; and there is undoubtedly much 
 evidence to support this opinion. 
 
 The second family of the Oynoidea is the Megalotidce, which is dis- 
 
 Temnocyon altigenis, Cope: i)art of Eight Jlandibuliir 
 Ramus, one-lialf natural size, viewed from without, 
 within, and above (after Cope). 
 
454 
 
 DE^^TAL ANATOMY. 
 
 tinguished from the Caniclw — and, for that matter, from all other 
 diphyodont monodelphous mammals — by the possession of fow true 
 molars in the lower jaw, thereby giving the formula I. ^, C. -^, Pm. 
 I", jNI. I" = 46. The only other cases in which there are more than 
 three true molars normally are found in the marsupials, edentates, and 
 cetaceans ; and in these two latter orders we have already seen that the 
 teeth are not generally divisible into incisors, canines, premolars, and 
 molars, on account of the development of only a single set. In the 
 marsupials, however, as we shall presently see, the normal number of 
 
 Fig. 232. 
 
 Skull oi Ailurodon s<mius, Leidy, three-eighths natural size (after Cope). 
 
 true molars is four, just as the number three is most common to dip- 
 hyodont monodelphs. Reduction of the normal number is to be fre- 
 quently observed in the monodelphs, and, as we have just seen in the 
 Canidce, occurs in genera otherwise nearly related ; it cannot therefore 
 be regarded as of more than generic importance, but there are no cases 
 known to me in which teeth have been added. On the contrary, I am 
 iirmly of the opinion that not so much as a single tooth has ever been 
 added to the diphyodont mammalian dentition in the course of develop- 
 ment, but that specialization has invariably gone in the opposite direc- 
 
TEETH OF THE VEBTEBRATA. 
 
 455 
 
 tion, as almost all evidence of palaeontology goes to show. The teeth 
 are not otherwise remarkable, resembling distantly those of the dog in 
 general pattern. The sectorials are not well defined, and the crowns 
 generally have a tendency lo the tubercular structure. 
 
 The second division of the Fissipedia [Ailuroidea) includes five fam- 
 ilies, the exact definitions of which the increasing knowledge of the 
 extinct forms is tending every ^ 
 
 day to break down into hopeless 
 confusion. The definitions are 
 already very unsatisfactory and 
 in many cases fail to define. 
 
 The families which approach 
 nearest to the Canidce are the 
 Hycenidce or hysenas, and the 
 
 Viverridce or civets. The evi- Superior Dental Series of Ictmerium robustum, two- 
 dence already cited brings the tWrds natural size (from Cope, after Gaudry). 
 
 former of these families into the closest relationship with the central 
 cynoid group. The dental formula of the existing hysenas (Fig. 234) 
 is, I. ^, C. \, Pm. 1^, M. -^ = .34. The incisors and canines have very 
 much the same pattern as the corresponding teeth in the dog, as do 
 also the premolars, with the exception of their more robust proportions 
 
 Fig. 234. 
 
 Skull of Striped Hysena, Hycena striata. 
 
 and the addition of an anterior cutting lobe to the superior sectorial. 
 In the lower sectorial the heel is very rudimental and _ the internal 
 tubercle is wanting. The single superior true molar is small and 
 
456 
 
 DENTAL ANATOMY. 
 
 Fig. 235. 
 
 Fig. 236. 
 
 situated just internal to the posterior part of the great superior sec- 
 torial, so as to be completely hidden in an external view of the jaw. 
 In an extinct species (Hyana eximia) there were four premolars in the 
 lower jaw, giving the formula I. J-, C. \, Pm. ^,M..\ — 36. The infe- 
 rior sectorial also has a well-defined heel. 
 In the more ancient or Miocene represent- 
 ative of this family {Hycenictis (jixvcd, 
 Fig. 235) the superior molar is much 
 larger and ha.s a more posterior position ; 
 the inferior sectorial (Fig. 236) has a rela- 
 tively large basin-shaped heel, and there is 
 a small second true molar behind it. It is. 
 through this genus that the transition is 
 etfected from the Hyaimke to the Canidce 
 
 Superior Sectorial and First Molar of by waV of IditllC rill ill and AUwodon. 
 
 /^,<...c«.,™c<. (after Gaudry). r^^^ ^^^^^^j formuk of the Vivcmke 
 
 varies somewhat by reason of decrease in number of the premolars 
 and molars. Tlie more important of these will be noticed after \\e have 
 first described the dentition of a typical example of the family, which is 
 found in the genus Herpestes, or the mongoose. The dental tbrmula is 
 
 I. f , C. {, Pm. A M. I = 40. The in- 
 cisors of the upper series have flattened 
 oval crowns without lateral lobes, in- 
 creasing in size from first to third ; the 
 canines are long, pointed, and recurved y 
 the first three premolars have the usual 
 pattern, but are devoid of accessory 
 cusps. In the fourth premolar or su- 
 perior sectorial the blade is composed 
 of the usual two posterior cusps, sepa- 
 rated by a fissure remarkable for its 
 depth. There is also a rudimental an- 
 terior basal lobe, Avhich arises from the 
 cingulum. The internal lobe is unusually strong, and sends a trenchant 
 ridge Ijackward and outward to join the principal cone. The next 
 tooth, or first true molar, is tritubercular, A\ith two external and one 
 internal cus]:) ; the crown is remarkalile for its transvei-se extent. The 
 last molar is relatively small, and has a more internal position, possess- 
 ing a bicuspid crown. The decrease in size of the true molars from that 
 of the great sectorial, and the strongly inward curvature of the tooth-line 
 beliind, are more pronounced than in the clog, and altogether interme- 
 diate between that of the latter animal and the cats. 
 
 The incisoi's of the lower jaw are smaller than the corresponding 
 te(th above, and the summits of their crowns are distinctly notched; 
 the canines are like those of the upper jaw, while the premolars have 
 basal cusps which are largest behind. The first true molar or inferior 
 sccttirial furnishes a pattern intermediate between the tuberculo-sectorial 
 and the well-defined sectorial. The primitive cone and anterior basal 
 lobes arc connected into a blade, the internal tubercle beina: large and 
 furnishing the characteristic triangular appearance of this portion of the 
 
 Fragment of Lower Jaw of If. grceca, show- 
 ing .--(-'ctorial and second molar (after 
 Gaudry). 
 
TEETH OF THE VERTEBBATA. 457 
 
 crown. The heel consists of a raised margin bearing several small tuber- 
 cles. The last molar is quadritubereular, and seems to have retained 
 the anterior triangle of the preceding tooth, together with one cusp of 
 the heel. If this be so, it is an exception to the general rule, according 
 to which the anterior cusp becomes obsolete. 
 
 In the two-spotted paradoxure {Nandinia) of West Africa the molar 
 series is frequently reduced to INI. ^, while in the bintourong {Ardidk) 
 the last molar above and the first premolar below are often absent. The 
 premolar formula of the genus Galidca is normally f , which likewise 
 obtains in the kusimanse (Crossa7-ohit.^) from tlie West Coast of Africa. 
 The form of the inferior sectorial of the genus Cynogale, a Bornean 
 representative of this family, is nearer that of a tubercular than a sec- 
 torial tooth. The three anterior cusps which go to make up the tri- 
 angular portion are very much reduced, and have altogether lost their 
 sectorial character ; the superior sectorial, liowever, is much better de- 
 fined as such. In another genus (^Eupleres) the teeth are very small 
 and the incisors stand far apart, on account of which, together with 
 several cranial peculiarities. Dr. Gill gives it a distinct family rank. 
 
 It will thus be seen in a survey of the dental organs of this family 
 that they are almost identical with the genus DidymioUs of our American 
 Eocene, which has already been described, and I think there can ))e little 
 doubt that they are the derivatives of this or some nearly related genus. 
 
 Another family, which stands intermediate between the civets and 
 cats, is represented by the single living genus Oryptoprocta, which is 
 limited in its distribution to the island of Madagascar. Some authors 
 classify it as a sub-family of the cats, others as a sub-family of the 
 civets, while others again make it a distinct family. No better argu- 
 ment, it seems to me, could be advanced in support of its intermediate 
 nature. It undoubtedly has strong affinities with both families, and 
 goes far toward bridging over the interval between them. The recent 
 discoveries of Cope and Filhol have shown it to be the surviving rem- 
 nant of an extensive group which lived in this country and Europe, and 
 which were the ancestors of the cats, and in all probability the deriva- 
 tives of the more generalized civets. In distinguishing between the 
 Felidce, Viverridce, and Cniptoproctidce the foramina at the base of the 
 cranium affijrd the best, if not the only, grounds for separation. Pre- 
 vious to our knowledge of the extinct forms the number of the molar 
 teeth was also used for this purpose, but owing to the intermediate 
 condition of this latter character in many of the fossils it must be aban- 
 doned as altogether worthless. In the Oryptoproctidce the alisphenoid 
 bone is perforated by a canal — the alisphenoid canal — for the passage 
 of the external carotid artery in its course forward. The foramen for 
 the entrance of the internal carotid in its passage to the brain is also well 
 defined, and of a considerable size. In the Felidw there is no alisphe- 
 noid canal, and the carotid canal is minute or absent. In the Viverridce 
 the alisphenoid canal is generally present, but not invariably so ; the for- 
 amen for the entrance of the internal carotid is of moderate proportions, 
 as in the Cryptoprootidce, from which I can see no very good reasons for 
 distinguishing them as a family. Cope associates a number of extinct genera 
 together under the name of Nimravidce, and defines them from the Felidce 
 
458 
 
 DENTAL ANATOMY. 
 
 by a number of characters in wliii'h they agree with the Oryptoproctidce ; 
 tlie distinctions bet-\veen them and this latter family are not so apparent. 
 
 The dental formula of Oryptoproda is I. f, C. \, Pm. -|, M. ^ = 34. 
 The incisors and canines resemble those of the cats generally ; the first 
 jjremolar in the upper jaw is caducous, and does not usually appear in 
 the adult skull. The superior sectorial has a rudimental anterior basal 
 lobe, an internal tubercle, and a well-defined blade. The molar is a 
 much smaller tooth, and has an internal jjosition, as in the hysenas. In 
 the lower jaw the sectorial has a iiiint heel and lacks the internal tuber- 
 cle, and is altogether feline in its appearance. 
 
 The following extinct genera are enumerated and defined by Cope as 
 belonging to the i&xmly Niinravidw.^ 
 
 I. Lateral and anterior faces of mandible continuous ; no inferior flange. 
 
 a. No anterior lobe of superior sectorial ; inferior sectorial with a heel ; 
 canines smooth. 
 Pm. I, JI. |-; inferior sectorial with internal tubercle . ... Procelurus. 
 
 Pm. I, jNI. I ; inferior sectorial without internal tubercle . . . Pseudcelurus. 
 Pm. I, JM. -f; inferior sectorial without internal tubercle . . Oryptoproda.'' 
 II. Lateral and anterior faces of mandible separated by a vertical angle ; no infe- 
 rior flange ; incisors obspatulate. 
 u. No anterior lobe of su]ierior sectorial ; inferior sectorial with a heel (and 
 no internal tubercle) ; incisiors truncate. 
 
 Pm. ■+, M. J; canine smooth Archcdurus. 
 
 Pm. ^, M. J ; canines denticulate . ... JElurogale. 
 
 Pm. I, M. |- ; canines denticulate ... . . .... Ximmviis. 
 
 III. Lateral and anterior faces of mandible separated by vertical angle ; an inferior 
 flange ; canines denticulate. 
 a. No or a small anterior basal lobe of superior sectorial ; inferior sectorial 
 with a heel. No posterior lobes on crown of premolars. 
 
 Pm. I, M. i . . . Dinictis. 
 
 Pm. j, M. ^ Pogonodon. 
 
 Pm. '"j- M. I Hoploplioneus. 
 
 Pm. Ti M- T • ■ ■ Eusmilm. 
 
 Prokeurus is known to have possessed five digits in each foot, as 
 O-yptoprocta, and it is probable that two sub-families should be made, 
 
 since others had only four in the 
 pes. The dentition of Procdurus 
 (Fig. 2.37) is more primitive than 
 Oryptoproda in the following cha- 
 racters : there are four premolars in 
 the lower jaw ; the superior sectorial 
 has no anterior basal lobe ; the in- 
 ferior sectorial has a strong heel and 
 an internal tubercle ; and there are 
 t)ro true molars below. 
 
 P.iciuJn'/iirus agrees more nearly 
 with Criijjtop7-oda, but lacks one 
 ]5remolar in the upper series. As 
 already ob.served, the first premolar 
 is caducous in this latter genus, and 
 Prote?M™.sii(/fen,-,rim., two-thirds natural size, they may be the same. In the sec- 
 
 ' "On the E-xtinct Cats of .\merica," Jmnienn Naturidid, Dec, 1880. 
 '' I have combined the Nhnratidce and the Cryptoproelidce, and have inserted this 
 genus where it seems to most appropriately belong. 
 
 Fig. 23 
 
TEETH OF THE VERTEBRATA. 
 
 459 
 
 ond section the anterior and lateral faces of the mandible are sepai'ated 
 by an angle or vertical ridge, which gives to the jaw the appearance of 
 having a square chin. 
 
 The genera of this section, with the exception of jElurogale, are from 
 the Miocene beds of the John 
 
 Day Valley, Oregon, and were ^. ^ Fig. 238. c 
 
 described by Cope ; the pre- mA " /mI 
 
 molar series shows a gradual re- 
 duction in number, but they all 
 retain the heel to the inferior c^j^ yys^^ ^^a^^ i« 
 
 sectorial and the generalized iVoretorasjM/i™!, rilh, two-thirds natural size; a, inner 
 
 1 . J, J J ° , . view of mandible: /j, superior view of inferior teeth; 
 
 character Ot two true molars in c, inferior sectorial, natural size (fiom Cope after 
 
 the lower jaw. Areilicelurus&i\di ^'"'°'^- 
 
 Nimravus are represented in the accompanying figures, 239 and 240. 
 
 Fig. 239. 
 
 Archaelurus Milts, Cope, Skull, one-half natural size (after Cope). 
 Fig. 240. 
 
 Skull of Nimravus gomphodiis. Cope, two-fifths natural size (after Cope): 1, 2, iirst and second true 
 molars ; 3, 4, third and fourth premolars of lower jaw. 
 
460 DENTAL ANATOMY. 
 
 In the third section the mandible possesses a strong inferior flange 
 upon each side to protect the powerful canines of the upper jaw, which 
 in some forms project far Ijclow the level of the symj^hysis. They are 
 therefore known as the " sabre-tooth division." In the first of these 
 genera, Dinidis (Fig. 241), the true molars are \, the inferior sectorial 
 
 Fig. 24]. 
 
 Skull of Dincitis Cyclops^ one-half natural size (after Cope). 
 
 has a heel, and the true molar above is a moderately well-developed 
 tooth, as in the preceding genera. The genera Hoplophoneus and Pogon- 
 odon carry dental specialization several steps further, while in Enymllus 
 we have the highest point reached by any of this group, which is in 
 many respects superior to the living cats. 
 
 Cope, in commenting upon the dentition of this group, says : " It is 
 readily perceived that the genera above enumerated form an unusually 
 simple series, rejiresenting stages in the folloAving modifications of parts : 
 (1) In the reduced number of molar teeth ; (2) in the enlarged .size of 
 the superior canine teeth ; (3) in the diminished size of the inferior 
 canine teeth; (4) in the conic form of the crowns of the incisors; (5) 
 in the addition of a cutting lobe to the anterior Ijase of the superior 
 sectorial tooth ; (6) in the obliteration of the inner tubercle of the lower 
 sectorial, and (7) in the extinction of the heel of the same; (8) in the 
 development of an inferior flange at the latero-anteripr angle of the 
 front of the ramus of the lower jaw ; (9) in the development of cutting 
 
 loljes u]3on the posterior border of the large premolar teeth 
 
 The succession of the genera above pointed out coincides with the order 
 
 of geologic time very nearly The relations of these genera are 
 
 veiy close, as they diifer in many casep. by the addition or subtraction 
 of a single tooth from each dental series. These c'liaractei-s are not 
 even always constant in the same species, so that the evidence of 
 descent, so far as the genera are concerned, is conelusivi'. No fuller 
 genealogical series exists than that which I have (list'o\'ered among the 
 extinct cats." 
 
 The last family of the Ailvroidea is the FcIifJa-, in -which we meet 
 with the highest point in specialization that has been reached iu the flesh- 
 
TEETH OF THE VERTEBRATA. 461 
 
 eating Mammalia. It includes two divisions — one in which the superior 
 canines are normal and without the vertical angles and inferior flanges 
 to the mandible; and another, "sabre-tooth division," wherein the 
 superior canines are enormously enlarged, denticulate, and protected 
 by inferior flanges of the rami. 
 
 The first of these groups or sub-families is the more generalized, and 
 embraces all the existing cats or those animals popularly known as lions. 
 
 Fig. 242. 
 
 Pogonodon plali/copis. Skull, less than two-fifths natural size, (afler Cope): 2, 3, 4, second, third, and 
 fourth premolars, and 1, first molar of lower jaw ; F G, post-glenoid foramen ; PP, post-parietal 
 foramen. 
 
 tigers, leopards, panthers, etc. Five genera have been establi.shed in this 
 division on characters of the teeth and orbit. It is here that the domestic 
 cat belongs, and its dentition may be taken as a good average represen- 
 tation of that of the sub-family. 
 
 The dental formula in this animal is I. f , C. \, Pm. |, M. \ = 30. 
 The incisors are relatively small, and are disposed almost transversely 
 across the fr«nt of the jaw. The first premolar above is a small, single- 
 rooted tooth, and is situated at a considerable distance from the canine, 
 which has the usual form and proportions of that tooth in the Carnlvora 
 generally. The second is larger and two-rooted, while the fourth 
 or upper sectorial is decidedly the largest tooth of the superior series ; 
 it has three external cusps united into a blade, and a small internal 
 tubercle. The single molar is very small and functionless, being 
 placed internal to the posterior part of the large sectorial. In the 
 lower jaw the premolars are proportionately large, having two fangs 
 and posterior accessory cusps. The sectorial is specialized, and con- 
 sists simply of two cusps forming a trenchant blade ; both the heel 
 and internal tubercle are absent. 
 
 The lynxes have one less premolar upon each side above than the cat, 
 and for this reason are placed in a distinct genus. In the flat-headed 
 cat and the fishing cat the orbit is completely encircled by bone — an 
 unusual occurrence in this family. In both, the number of teeth is the 
 
462 
 
 DENTAL ANATOMY. 
 
 same as in the domestic eat, but in the former the first premolar in the 
 upper jaw has a single fang, whereas in the latter this tooth is two- 
 rooted. Upon these characters two genera have been established. The 
 clouded tiger of India has a dental formula like that of the lynxes, and 
 approaches the " sabre-tooth division " in the enlargement of the supe- 
 rior canines, by reason of which it has also been given a generic rank. 
 The hunting leopard, or cheetah, forms another genus, and is distin- 
 
 FiQ. 243. 
 
 Cranium of Smilodon necaloi-, Gervais, one-tMrd natural size (after Cope). 
 
 guislied by the absence of the internal tubercle of the superior sectorial. 
 All the other cats are very much alike, and can be distinguished from 
 one another only specifically, being classified, therefore, under the genus 
 Felis. 
 
TEETH OF THE VERTEBRATA. 463 
 
 The second division is extinct, despite the fact that they reveal to us 
 the most perfect laniary dental apparatus yet known within the limits 
 of the Carnivora, and were of the most formidable size. Two genera 
 are known, of M'hich the cranium of one (Smilodon) is represented in 
 Fig. 243. In this animal the dental formula is I. f, C. \, Pm. ^^ri, 
 j\I. ^ = 24 or 26, and marks the extreme point in dental specialization in 
 this order, as far as reduction is concerned. The canines of the upper jaw 
 are of prodigious size in comparison with those of the lower series, having 
 compressed crowns with serrulate edges. The sujjerior molar has disap- 
 peared, and the first premolar in the lower jaw in some species is 
 wanting. The exact use of the great superior canines is not very 
 clearly understood. The possession of retractile claws, the reduction of 
 the molar and premolar series, together with the general perfection of 
 the sectorial apparatus, are strictly in keeping with a most carnivorous 
 habit ; but with all this it must have been impossible for the animal to 
 open its mouth wide enough to take a firm grip upon a living prey, on 
 account of the great length of the upper canines. 
 
 Seeing that in the existing cats their chief destructive powers reside 
 in their biting qualifications, it is difficult to understand how these 
 animals inflicted wounds sufficient to destroy their prey, unless they 
 did so with that part of the tusk which projected below the level of the 
 symphysis when the mouth was closed, just as the walrus uses his tusks 
 to clamber over the ice. They may also have been used to assist the 
 animal in climbing, and in this way attained their great size. 
 
 The animals composing the last group, Ardoidea, are the least car- 
 nivorous, and do not as a general rule display as trenchant and sectorial 
 dental organs as the two preceding ; in two families the almost exclu- 
 sively carnivorous habits are manifested by sectorials of moderate per- 
 fection ; this condition is associated with a reduction of molars and 
 premolars from the number possessed by the dog. In the others the 
 molars are more or less tubercular — a structure better fitted for the 
 mastication of the mixed diet upon which they subsist — and usually 
 exceed the premolars in size and strength. The extremes of dental 
 variation in this group are exhibited by the bears and weasels, of 
 which the former are the farthest and the latter the least removed 
 from the more typical carnivores in the structure of the teeth. 
 
 In the bears the dental formula is the same as in the dog, but in 
 most of the living species the three anterior premolars are very small, 
 and frequently disappear in old age, leaving a wide space between the 
 fourth and the canine. In the upper jaw the teeth progressively 
 increase in size from the fourth premolar to the last molar, which, 
 besides being quadritubercular, is provided with a large posterior 
 heel rounded off behind ; by the addition of this heel the crown is 
 rendered elliptical in transverse section, the antero-posterior diameter 
 being twice that of the transverse. The first true molar has four 
 cusps on its triturating face, and is subquadrate in outline ; the fourth 
 premolar is tricuspid, as in the dog, but the two outer cusps are not 
 united into a perfect blade, and the internal lobe is large and has a 
 median position. This tooth is relatively small, and is situated consid- 
 erably in advance of the canthus or angle of the mouth ; it is doubtful 
 
464 DENTAL ANATOMY. 
 
 ■whether its possessor ever makes use of it as a sectorial organ, but 
 rather prefers to tear the tough animal membranes than to divide them 
 with the sectorials, as the dogs and cats do. 
 
 In the lower jaw the first true molar betrays the same lack of car- 
 nivorous specialization as the upper teeth, being essentially tubercular 
 in structure, although the jii'oper elements of the sectorial of the dog 
 can be easily made out ; the crown is much elongated, and is narrower 
 in front than behind, the heel composing at least half of the crown. 
 The next tooth behind it is the largest of this series, and is perfectly 
 quadritubercular ; the last molar is smaller, with a subcircular grinding 
 face, upon which the tubercles are poorly defined. 
 
 While the structure here described is found in all the northern more 
 carnivorous bears, the tropical frugivorous species retain to a greater 
 extent the integrity and more normal condition of the anterior premo- 
 lars. This is especially apparent in a genus recently discovered in the 
 mountains of oriental Thibet and described under the name of-^Eluro- 
 piia. In this animal the first premolar only is small, Avhile the others 
 gradually increase in size to the last molar, which has a comparatively 
 small heel. 
 
 The palseontological evidence is as yet too meagre to demonstrate 
 with any considerable degree of certainty the evolution of this group of 
 the Cttrnivora, but some suggestive hints of their former connection with 
 the Ci/noidea are afforded by the extinct genus Hija'naivtoK, AA-hich -was 
 originally described by Dr. Falconer from the Sewalik Hills in India. 
 This genus displays three premolars of normal projDortions and a large 
 sectorial, together with a last superior molar in which the heel is absent. 
 In one species in j)articular, H. hemicyon, from the Miocene of Sansan 
 in France, which is provisionally referred to this genus, the two true 
 molars in the upper jaw have about the same projjortions as in the dog, 
 and otherwise resemble them very much. The sectorial, as is indicated 
 by the roots, was large, with the internal tubercle placed opposite the 
 middle part of the crown. If it were not for this latter fact, the frag- 
 ment of the upper jaw upon which the species was established would 
 readily pass for that of a member of the Canidfr. 
 
 In the weasels, which constitute the family J/uKtc/ula', the sectorials 
 are well defined as such, and some of them, notably the typical weasels, 
 piissess retractile c]a\^•s. In none docs the molar formula exceed ii-, 
 except- a fossil genus, Lniridis, a near ally of the otters, in MJiich the 
 molars are two in the upper jaw ; it may, however, be reduced to \, as 
 in the case of the Cape ratel (JTcIIiroi-d capensi.^). The premolars vary 
 in number, as do alstj the sectorial in structure. 
 
 The dentition of the American pine marten (Fig. 244) will serve 
 as an illustration of this family, althoutih it is somewhat more 
 specialized in a carnivorous direction than most of them. Its dental 
 formula is I. f, C. {, Pni. A, M. l = 38. The incisors, canines, and 
 premi liars have approximately the same structure as those of the dog, 
 except that the fourth premolar or superior sectorial has the two outer 
 cus])s blended together, with the vertical notch absent. The true molar 
 is tubercular, and has a greater transverse than longitudinal extent. 
 In the lower jaw the sectorial is very much like that of the dog, 
 
TEETH OF THE VERTEBRATA. 
 
 465 
 
 Fig. 244. 
 
 while the second molar is small, single-rooted, having a crown with 
 one cusp. 
 
 The teeth of the raccoons and allied forms are intermediate between 
 those of the bears and weasels in many respects, with a stronger tend- 
 ency to the tubercular than to the sectorial pattern. 
 
 Teeth of the Cheiroptera. — The modification of the anterior 
 members for flight distinguishes tliis order from all other unguiculates 
 at once. Excluding this peculiarity, which is universal among them, 
 they are closely related to the Inseetivora, 
 and without doubt have been derived 
 from some arboreal representative of 
 this order. It is conceivable that in 
 jumping from branch to branch they 
 have first developed a lateral fold of 
 integument, similar to that seen in the 
 flying squirrels, which later involved 
 the fore limbs and extended to the neck. 
 The flying lemur (Galeopithectis) fur- 
 nishes such a transitional condition, both 
 in the possession of the membrane and 
 the elongated and slender fore limbs, al- 
 though it is highly improbable that the 
 bats have descended through this genus. 
 
 The incisors are never more than two 
 upon each side above, while the lower 
 jaw is usually provided with the same 
 number, but may be increased to three. 
 Canines are always present in both jaws, 
 but are of variable proportions. In the insect-eating forms (An imalicora), 
 which includes the great bulk of the species, the upper molar teeth invari- 
 ably display the peculiar W-pattern of the moles, shrews, etc. of the Insee- 
 tivora, already noticed. The premaxillary bones are always small, and 
 seldom meet in the median line so as to leave the tooth-border inter- 
 rupted in front. In the W-pattern of the superior molars, the absence 
 of the median pair of upper incisors, and the small premaxillaries it is 
 interesting to note the resemblances they bear to the squirrel shrews 
 (Taupaiadce), the only other insectivores besides the flying lemur which 
 are known to be arboreal in habit. Ignorance of the rest of the anat- 
 omy of this genus does not permit me to state whether it strengthens 
 this resemblance or otherwise, but upon the M'hole I am inclined to 
 believe that some such arboreal insectivore was the ancestor of the bats. 
 
 The dentition of the blood-sucking vampires is modified in accord- 
 ance with their habits, as is also the entire alimentary canal, and devi- 
 ates quite extensively from the normal condition of that of the insect- 
 eaters. The alimentary tract consists of little more than a straight tube 
 from mouth to anus, and is thus adapted to the assimilation of the 
 blood of living animals, upon which it feeds. 
 
 The large incisors of the upper jaw are two in number, one upon each 
 side, whose roots extend into the maxillary bone, and whose compressed, 
 sharp-pointed, hook-shaped crowns are specially fitted to puncture the 
 
 Vol. I.— 30 
 
 Vertical View of the Upper and Lower 
 Jaw of American Pine Marten [Mustela 
 ameHcana). 
 
466 DENTAL ANATOMY. 
 
 skin of an animal sufficiently to cause the blood to flow freely. The 
 canines are almost equal in size and similar in shape, while the lower 
 incisors and canines are small. The molars are reduced to two in the 
 upper and three upon each side in the lower jaw ; the upper molars are 
 implanted by single fangs and have simple conical crowns ; in the lower 
 jaw the first Uvo are like those above, but the third has two fangs and 
 a bilobed crown, and is considered by Owen to be homologous with the 
 last j^remolars of insectivorous bats. The dental formula is thus re- 
 duced to I. 1, C. {, Pm. I = 20. 
 
 The Irugivorous bats, which are popularly known as " flying foxes," 
 offer another deviation from the usual structure in the pattern of the 
 molar teeth ; those in the upper yaw have crowns of a subeircular form 
 in outline with a central longitudinal depression, upon each side of 
 \\hich the edge is elevated into a cusp. Those of the lower series are 
 similar but smaller, with the cusps more pronounced and the median 
 groove narrower. 
 
 Teeth of the Rodentia. — The amount of minor variation in the 
 dental organs of this order is so extensive that their complete elucida- 
 tion is hardly within the scope of the present work ; a description of 
 the leading types must suffice. That which most conspicuously distin- 
 guishes the rodents from all other mammals is the possession of two 
 powerful curved incisors in each jaw, which grow from persistent pulps 
 and are faced with enamel. The roots are implanted deeply in the sub- 
 stance of the jaw bones in the lower jaw, often reaching as tar back as 
 the coronoid process. In consequence of the distribution of the enamel 
 upon the front face of the tooth, leaving the dentine naked behind, the 
 inc(|uality of wear between the two surfaces is always marked, and C(jn- 
 stantly preserves a chisel point to the crown — a structure pre-eminently 
 adapted to the gnawing habits of its possessor. Concomitant with this 
 modification the canines are always absent, the preniaxillar}' bones are 
 large to support tJie roots of the incisors, and there is a wide space 
 between the first molar or premolar and the incisor, in which no teeth 
 appear. The mandibular condyle, moreover, is globular in form and 
 never trans\-erse, thereby allowing excursion only in an antero-poste- 
 rior direction. 
 
 The order thus distinguished is divisble into four sub-orders, of 
 which the rat, squirrel, porcupine, and rabbit are typical representatives 
 of each. 
 
 The dental formula of the common rat (Fig. 245) is I. ^, C. ^, Pm. ^, 
 
 M. -1 = 16. Deciduous teeth are entirely 
 i&- ^4&. wanting, and it is therefore monophyodont 
 
 — a condition \vhich we would be led to 
 anticipate, as far as the molar and premolar 
 series is concerned, in view of the subtrac- 
 tion of the latter. The absence of any de- 
 craiiium of rominon Eat, .V".< eiduous prcdcccssors of the two pairs of in- 
 cisors IS said to be a constant feature of all 
 rodents except the hares, so that monophyodontism of this highly heter- 
 odont animal need not occasion surprise. 
 
 The incisors are of the usual pattern displayed by the order — large. 
 
TEETH OF THE VERTEBBATA. 
 
 467 
 
 curved, compressed teeth, with chisel-shaped crowns, which are stained 
 a deep orange color on the anterior face ; the })igment which produces 
 this color is intimately incorporated witli the enamel itself, as in the 
 shrews, and serves to sharply define the limits of the enamel covering. 
 
 In both the upper and lower jaws the first molar is the largest and 
 the third the smallest. They are implanted by distinct roots, the oppo- 
 site rows of teeth being nearly parallel. The crowns are made up of 
 three curved transverse ridges, with the con\-exity in front and the con- 
 cavity behind ; the two anterior of these, in the upper teeth, are termi- 
 nated internally by well-marked cusps, which rise above the summits 
 of the ridges. In the last tooth the anterior and posterior of these 
 ridges are less distinctly marked, and are reduced to little more than 
 internal tubercles. The second molar of the lower jaw has the last 
 crest rudimental, and in the third it is entirely wanting. 
 
 While this structure prevails in the teeth of the more typical murines, 
 others possess molars with crowns of much greater complexity and 
 without roots. Such is exemplified by the arvicoline 
 section of the Muridoe, in which the crown is cleft to 
 the median line by vertical fissures upon each side 
 placed alternately. The structure of the grinding sur- 
 face which results from this arrangement is a system 
 of alternate triangular prisms connected in the middle 
 of the crown by a narrow band of dentine. This is 
 well shown in the accompanying figure. 
 
 In the S(][uirrel division premolars are always pres- 
 ent, in consequence of which there are deciduous teeth. With the excep- 
 tion of the beaver family, the teeth are very similar in the different spe- 
 cies, the only important variation occurring in the number of premolar 
 In the common fox squirrel the dental formula is I. \, C. ^, Pm. 
 M. f = 20. The incisors (Fig. 247) are not so robust as in the rat, 
 and, like them, are colored upon the anterior face. 
 
 Fig. 247. 
 
 Fig. 246. 
 
 Vertical View of the 
 Grinding Surface of 
 tlie First Lower 
 ^lolar of a Muskrat 
 {FWer zibelhiciu). 
 
 1 
 
 Vertical View of the Teeth of Fox Squirrel (Sehirm caroKnensis). 
 
 The first and only premolar is smaller, implanted by three roots, and 
 has a triangular tricuspid crown. The three true molars in the upper 
 
468 DENTAL ANATOMY. 
 
 jaw are larger and subequal in size. Their crowns are imperfectly quad- 
 rate in outline, and provided with U\i\ transverse crests which join a 
 large marginal cusp on the internal border. There is in addition an 
 anterior and posterior cingulum, which becomes continuous with the 
 lar!j;c marginal cusp. The inferior molars have the same quadrate out- 
 line as those above, but the crowns present a central depression sur- 
 rounded by a sliglitly elevated margin bearing a cusp at each angle. 
 In this sub-order is to be found the nearest approach to the quadri- 
 tubercular condition of the molar teeth in any of the liodeiitia, in con- 
 sequence of whicli there is little difScultyin I'omprehending tlieir organ- 
 ization ; but when \ve come to analyze the highh' complex form of 
 molar which some of the porcupines exhibit, we naturally seek for a 
 key to a solution of their structure on the basis of the quadritubercular ; 
 this is all the more natural ^vhen we remember that the squirrels present 
 the oldest known representati\es of the order in the genus Pk.siarciotnys 
 of Middle Eocene Age, which scarcely differs generically from the living 
 forms. The teeth of the American porcupine {Erethhon), while possess- 
 ing in general the molar pattern of the squirrel, nevertheless differs from 
 it sufficiently in the diret'tion of the more specialized hystricine teeth to 
 let us into the secret of how these complex forms have arisen from that 
 of the squirrel. 
 
 In the description of the molars of the squirrel we have already seen 
 that the face of the crown is marked by three transvei'se ridges, enclos- 
 ing two valleys, Avhic'h (i2)en externally and are bound- 
 ed internally l)y a thick marginal cusp. X(.iw, in the 
 first j^remolar of the porcupine (Fig. 24S) which is un- 
 usually instructive, the three transverse ridges are pres- 
 ent, but considerably augmented in height, together 
 Avith a fourth ridge behind added from the cingulum. 
 The valleys separating the three anterior crests open 
 external]}', Mhile the fourth coalesces externally M'ith 
 the third, s(( as to enclose a deep pit or fossette ; the 
 C strong internal marginal cusp is likewise present, but 
 
 "^ofpOTcTfpinr'i&rf/li- '^ interrupted on its inner side by a deep wide valley 
 zoM thrsaii.-^), vertical \\-hich opcns internally. The succeeding molars are 
 posterior; c, internal; like it in structure, cxccpt that the first and second 
 faces "of ''tiircrow™'^" trausvcrsc ridges unite at their extremities to form 
 a second fossette in front. 
 In other genera of this group the valleys are still further deepened by 
 the elevation of the i-idges, and other indentations are added from within. 
 As a protection against fracture of the now laminar crests, cementum is 
 added, which completely fills up the valleys, leaN'ing the grinding surface 
 approximately smooth. This is the condition attained by the bi'aver 
 among the sciuromorph or squirrel sub-order, as well as a majority of the 
 hystricomorphs or porcu]3ines. As a further complication in this series, 
 the external and internal valleys unite across the face of the crown, leaving- 
 transverse laminte connected oidy at the base and bound together above 
 by cementum ; of which the guinea-pig is an exanqjle. Finallj', the 
 extreme of specialization is reached in the capybai-a, M-herein these trans- 
 verse laminae are as many as thirteen or fourteen in a single tooth. 
 
TEETH OF THE VERTEBRATA. 469 
 
 This point of perfection rivals that of the elephant, and is undoubtedly 
 a long way removed from the quadritubercular structure. On account 
 of this highly complex molar dentition and certain cranial pecidiarities 
 Dr. Gill has proposed to give this genus a distinct family rank. 
 
 The last sub-order of the Rodenlid is the Lagomorpliu, \\hich includes 
 the hares and rabbits. The dental formula in this group is constantly 
 I. I", C. ^, Pm. I, M. 1^ = 28, in addition to which in very young spe- 
 cimens there is another or third pair of inci- 
 sors in the upper jaw to be added to the per- ^^'^- 249. 
 raanent set. Huxley has recently shown that 
 the deciduous dentition is D. I. f, D. M. '_ 
 which brings this group of the Rodentla into 
 strict accord with other Mammalia in the re- 
 placement of the teeth. Last Molai- of Capybara fHydro- 
 ^ _,. . f ... -. chtxnts capybara), vertical view. 
 
 ihe median pair ot superior incisors depart 
 from the usual pattern, inasmuch as they are indented upon their anterior 
 faces by a vertical groove near the middle of the tooth ; they are other- 
 wise as in the genera already noticed, except that they lack the orange 
 color of the enamel. Immediately behind each of these incisors, and 
 applied closely to them, is to be seen a small cylindrical tooth, the second 
 pair of incisors. In the very young state a third pair can usually be 
 found imbedded in the gum external to the two median ones, which fall 
 out soon after birth. The single pair of the lower jaw are not grooved 
 and have the usual form common to the order. 
 
 The molars are remarkable for their great length in a vertical direc- 
 tion, as well as their antero-posterior compression ; they grow c( )ntin- 
 uously and do not form roots. With the exception of the first premolar 
 and the last molar, the molars and premolars of the upper jaw are alike, 
 and consist of two vertical transverse laminae closely united in the middle 
 line, the division of which is indicated both on the inner and outer sides 
 of the tooth by a vertical groove. The first premolar and last molar are 
 made up of a single lamina, the enamel being thrown into two vertical 
 folds upon the anterior part of the first premolar. In other respects the 
 rabbits are remarkable for the entire absence of the coronoid process 
 and the very small bony palate, which forms little more than a bridge 
 across the roof of the mouth. 
 
 Teeth of the Ungulate Series. 
 
 So far, excluding the rodents, our attention has been confined to those 
 dental organs in which the molars have not, with few exceptions, passed 
 beyond the quadritubercular stage of development ; this condition, we 
 have the best of reasons to conclude, was preceded by the tritubercular 
 in the upper and the tuberculo-sectorial, or at least a tooth possessing 
 its elements, in the lower jaw. When one compares these short-crowned 
 rooted tubercular molars with the complex rootless molars of a horse, 
 cow, or elephant, he might spend hours and days in thoughtful contem- 
 plation without discovering the faintest relationship existing between 
 their respective patterns ; nor would we be any nearer a solution of 
 the difficulty had not the researches of palaeontologists brought to our 
 
470 
 
 DENTAL ANATOMY. 
 
 understanding a knowledge of these organs before they had assumed those 
 distinctive characteristics and specialized patterns which they now display. 
 
 There are few students of odontography who are acquainted with the 
 facts of mammalian palseontology as they now stand who have not had 
 repeatedly forced upon their attention the gradual decrease in complexity 
 of the molar teeth of the ungulates as we go backward in time. Cope 
 has recently shown that the earliest ungulates had, as a general rule, 
 tritubercular molars — a condition which is as primitive as that of many 
 insectivores ; and in no instance do \\e meet with highly specialized teeth 
 until the latest geological periods are reached. 
 
 The ungulate series is divisible into four orders, which have been 
 characterized and defined Ijy Cope upon the structure of the limbs. The 
 oldest of these orders, Taxmpoihi, is remarkable for the generalized cha- 
 racter of the limbs as compared with the later ungulates ; they possess five 
 toes upon each foot, and in one family, the Periptychidce, the superior 
 true molars are tritubercular — a fact which brings the ungulate stem to 
 a point not far removed from the Insectivora of the unguiculates. This 
 order includes three sub-orders, two of which are extinct, and one, the 
 Hyracoidcu, being represented by two living genera, popularly known 
 as the coneys. The most ancient of these three sub-orders is the Con- 
 dylarthra, a group thus far known only from the American Eocene. A 
 careful study of their osteology leads to the conclusion that they are the 
 ancestors of all succeeding ungulates, furnishing just such a generalized 
 type in the proper geological position as is necessary to satisfy the 
 demands of the development hypothesis ; thej' likewise enable us to 
 comprehend more clearly the mutual relationship and evolution of the 
 entire series. 
 
 Teeth of the Taxeopoda. — As the Condylarthra are the oldest 
 of this order and the most primitive in their organization, it will be best 
 
 Fig. 250. 
 
 Dentition of Periptychus rhabdodon, Cope, two-thirds natural size; a, superior molars from below; 
 b, iuferior molars from above— (rom the New Mexican Puerco (alter Cope). 
 
 to commence with a consideration of their teeth. Three families are 
 referred to it, one of which, the Periptychidce, is confined to the lowest 
 Eocene deposits.^ In tlie typical genus, Fei-iptychus (Fig. 250), the 
 
 ' AVheii the lowest Eocene is mentioned, reference is made to the Puerco beds, which 
 were formerly considered to belong to the Tertiary ; Prof. Cope now considers that they 
 are of Cretaceous age. 
 
TEETH OF THE VERTEBBATA. 
 
 All 
 
 dental formula is I. f , C. \, Pm. f, M. f = 44, the normal diphyodont 
 number. The incisors are relatively small and of the usual pattern ; the 
 canines are large, powerful teeth, and resemble those of many carniv- 
 orous and insectivorous animals. The premolars gradually increase in 
 size fi'om the first to the fourth, which considerably exceeds the true 
 molars in size ; the crowns of the last three premolars in the upper ja\v 
 have a large external conical cusp and a strong internal ledge ; those of 
 tlie lower jaw have a strong outer cusp, with a small accessory one at 
 tiie antero-internal, and two at the postero-internal, angle of the crown. 
 The true molars of the upper series appear at first sight to be com- 
 plex and multicuspid, but upon analysis it is found that they are essen- 
 tially tritubercular, ^vith. minor cusps added. The tA\'o usual external 
 cusps are present, together with one large internal tubei'cle somewhat 
 crescentic in horizontal transverse section. The three principal cusps 
 are homologous with the three cusps of the molar teeth of many of the 
 Insectivora already mentioned, and like them are placed in the form of 
 a triangle, but the two horns of the crescent are interrupted by the 
 development of two intermediate cusps ; to these are added two small 
 interior cingnlar marginal cusps, making seven in all. The lower 
 molars are quadritubercular, with a faint representation of the anterior 
 basal cusp of the tuberculo-sectorial still remaining. The postero- 
 external cusp is connected with the antero-internal by a ridge which 
 crosses the face of the crown obliquely ; this ridge is found in some of 
 the insectivores, notably Esthonn.r, and is what remains of the former 
 connection of the heel with the anterior or triangular part of the tuber- 
 culo-sectorial. The enamel of both the molars and premolars of this 
 genus is curiously sculptured, owing to the presence of a number of 
 
 Fig. 251. 
 
 Ecioconus diti — ", maxillary and premaxillary bones from below, 
 
 retaining a" good deal of the matrix; b, last two inferior molars, worn by use; c, three deciduous 
 with first permanent molar of a young animal (after Cope). 
 
 vertical grooves and ridges, it being the only case of the kind known in 
 the Mammalia. In an allied genus, Edoganus (Fig. 251), the molars 
 are larger than the premolars, and their crowns are further complicated 
 by the addition of an outer cingnlar cusp, giving a total of eight of the 
 
472 DENTAL ANATOMY. 
 
 most complex tritubercular teeth yet known. This figure displays 
 more clearly than that ot Peiijjtyclms the relationship of the component 
 cusps. 
 
 Other genera of this family, of which there are seven in all, display 
 simple tritubercular molars, ^^'hich resemble the corresponding teeth of 
 the insectivorcs to a remarkable extent. 
 
 The second family uf this sub-order is the PhenacodontiJcc, which 
 continues to the Upper Eocene Period. Fragmentary remains of the 
 typical genus Phenacodiis were known as long ago as 1873, but very 
 little was known of its true nature until, some nine j'cars later, the 
 writer was fortunate enough to discover two almost complete skeletons, 
 representing t^\'o distinct species, in a fine state of preservation while 
 exploring the ^\'asatch deposits of the Big Horn Basin, Wyoming Ter- 
 ritory. This material has afforded Prof. Cope, at whose instance the 
 exploration Mas undertaken, the opportunity of not only determining 
 the position and affinities of this remarkable genus, but a key to a cor- 
 rect interpretation of many of his later discoveries, as well as a basis for 
 one (jf the most important generalizations yet introduced in relation to 
 the hoofed Mammalia.' 
 
 The dentition of this genus (Fig. 252) approaches nearer to that of 
 the higher ungulates than the preceding family, although the interval 
 between them is comparatively small. Its formula is I. f, C. ^, Pm. ^, 
 ^1. 1^ = 44. The premolars are of a simpler pattern than the molars, 
 the posterior ones becoming tritubercular. The superior molars have 
 quadrate crowns bearing four principal cusps, placed at each angle, to 
 Avhich are added several minor cusps, the rudiments of structures which 
 assume considerable importance in the later and more sjjecialized gen- 
 era. The four principal cusps are the usual ones of the quadritubercular 
 molar, two external and two internal, and are low, more or less conic, 
 obtuse structures. Between the outer and imier ones are two isolated 
 tubercles, which are later developed into cross-ridges connecting the 
 outer and inner cusps, thereby producing the lophodont molar -^vhich 
 is so characteristic of some groups of the ungulates. At a point mid- 
 way between the two outer cusjis, on the external margin of the crown, 
 the cingulum is produced into a small tubercle, which in most of the 
 specialized ungulates becomes connected with and unites the two Vs 
 formed by the crescentic structure of the tw^o external cusps, just as in 
 some of the insectivorous genera already described. 
 
 In the lower molars four tubercles are present, of which the jiostero- 
 external is connected ^^-ith the antero-internal liy a well-mai'ked ridge. 
 The anterior basal lobe is reduced, but still present in the form of a low 
 cingular ridge. 
 
 The molar teeth of this animal display a typical bunodont dentition, 
 and upon a correct understanding of their organization depends a proper 
 comprehension of all the succeeding specialized molars of this series. 
 It is by simple additions to, and modifications of, the component lobes 
 and crests of this jiattern that all the conq^lex ungulate molars have been 
 produced; if the advocate of the evolution hypothesis had no other evi- 
 
 ' iSee Prof, fripe's valuable memoir of this group, American Naturalist for August 
 and September, 18S4. 
 
TEETH OF THE VEBTEBBATA. 
 
 473 
 
474 
 
 DENTAL ANATOMY. 
 
 dence upon which to base his belief than that afforded by the gradual 
 complication of the molar teeth from this point upward in the hoofed 
 
 Fig. 253. 
 
 Mertiscoiherhim terxETubjce, Parts of Cranium, three-fourths natural size — from Wasatch Beds of New 
 Mexico; a, cranium fiom above; b, Irom below; f, portion of upper jaw, displaying deciduous 
 molars (after CopeJ 
 
 Mammalia, this alone, it appears to me, would be sufficient to gain for 
 it a respectful consideration at the hands of its opponents. 
 
 Fig. 254. 
 
 Lower Jaw of J/. terrcBt'ubrre, three \ iuw s (after Cope). 
 
 The last family of this sub-order is tlie Meniscoiheriidce, whose den- 
 tition is represented in Figs. 25.'3, 254. The dental formula of the 
 
TEETH OF THE VERTEBBATA. 475 
 
 single genus Menisooihenum is given by Cope as follows : I. f, C. \, 
 Pm. 1^, M. -I = 44. As compared with I'hcnacodm, the canines are 
 relatively smaller and the molars more complex ; the same elements arc 
 readily recognized as in the molars of that genus, but the two exter- 
 nal cusps are crescentic and elevated, the two contiguous horns being 
 connected with the median external cusp, which now forms a vertical 
 ridge or rib on the external jjart of the crown. The intermediate tul)cr- 
 cles are also present, and are greatly enlarged ; the anterior is crescentic 
 and the posterior oblique and elongate. Of the two internal cusps, the 
 anterior is conic, while the posterior is crescentic. 
 
 The lower molars exhibit two Vs, by reason of the development of 
 cross-ridges connecting the external with the internal cusps and the 
 increase in height of the oblique ridge. The tooth-line is uninterrupted 
 by a diastema, and the incisors did not groM' from persistent pulps. 
 
 The second sub-order, Hyracoidea, has long remained a puzzle to 
 zoologists, and has been associated at different times near the rodents, 
 at others with the perissodactyle ungulates, and latterly has been made 
 the type of a distinct order. The discovery of the Condylarthra leaves 
 no doubt of its relationship with these forms, and the propriety of 
 making it a sub-order of the Taxeopoda is at once apparent. The 
 dental formula of the two living genera, Hyrax and Dcndrohyrax, is 
 given, I. f , C. ■§-, Pm. |^, M. | = 36, although DeBlainville in his figures 
 of the different species represents some of them with only two incisors 
 in the Upper jaw instead of four. 
 
 The incisors grow from persistent pulps and have large pointed 
 crowns ; the canines are entirely absent from both jaws ; the molars 
 and premolars have complex crowns — in one genus, Hyrax, being 
 almost identical with those of the rhinoceros ; on account of this com- 
 plexity Cuvier placed it in the same group with that animal. In the 
 other genus, Dendrohyrax, the molar teeth are quite different, and upon 
 careful comparison with those of Meniscotherium betray unmistakable evi- 
 dence of near relationship. This resemblance is p^^ 255 
 not confined to the molar teeth alone, but is strik- 
 ingly shown in the general form of the skull, 
 and especially in the great enlargement of the 
 angular portion of the mandible. The upper 
 molars of this genus (Fig. 255),^ like Menisoo- 
 therium, have two crescentic external cusps con- 
 nected by a vertical rib, and two intermediate ^f/„Jf '^■erUcarvtewf ^ 
 tubercles, which are more or less blended with ll'rX'oT.Tii^ternaran^ 
 the two internal cusps, while the lower molars *! posterior ' surfaces of 
 
 . r ' crown; b, inferior molar; 
 
 have essentially the same pattern as those ot 1, external, 2. internal sur- 
 this genus. The whole structure of the molars ''''^ '^^''^ ^^ '''^'"^■^"^)- 
 represents just such an advance over that of the extinct Eocene genus 
 as we should be led to anticipate on a priori grounds. 
 
 It is true that the canines are absent in Dendrohyrax, and incisors 
 
 o-row from persistent pulps ; but this is not at all remarkable when we 
 
 consider the great interval of time between them and an approach to 
 
 this condition, as far as the canines are concerned, in their reduced size, 
 
 ' This figure does not represent the structure of the grinding face very clearly. 
 
476 DENTAL ANATOMY. 
 
 in the extinct genus. Altogether, I am disposed to regard Menisco- 
 therium as the direct ancestor of the Hyracoidea, notwithstanding tlieir 
 wide separation in both time and space. 
 
 As a further comj)lication in the molar pattern of this line, ^\e have 
 the complete fusion of the intermediate tubercles with the internal cusps 
 in Hi/ra.r, \\hich, as already stated, gives the pattern of the molars of 
 the r]nnoccr(js. 
 
 It is believed l)y Cope, from evidence afforded Ijy the structure of the 
 limbs, that the Toxodontia, a group of curious extinct ungulate forms 
 found in the later geological horizons of .S( )uth America, belong to this 
 order. I am unable to find any confirmation of this position from a 
 study of the teeth, but it may be that they have been derived from a 
 condylarthrous source.^ 
 
 The dentition of the typical genus Toxodon contains incisors, pre- 
 molars, and molars only, the canines being absent, and all were of per- 
 sistent growth. The two pairs of incisors above are large and scalpri- 
 form, as in the rodents, of which the outer greatly exceed the mesial 
 pair in size. In the lower jaw these teeth are three in number upon 
 each side, and were also of persistent growth. They are subequal in 
 size, and have imperfectly prismatic crowns similar in shape to the tusks 
 of the boar in transverse section, being covered with enamel only upon 
 the anterior convex surface. 
 
 The molars, of which there are seven upon each side above, gradually 
 increase in size from the first to the last. It is highly probable that 
 the first four of these teeth are premolars, but in the absence of any 
 knowledge of the milk dentition and the manner of its replacement, 
 this, of course, is inferential. They have remarkably long crowns, 
 with an altogether unique pattern, and did not develop roots. In 
 section they are triangular, with the apex of the triangle directed forward 
 and outward. Upon the inner side there is a deejj indentation or fold 
 reaching to a point near the centre of the crown, which may be the 
 valley separating the two internal cusps. The only arrangement sim- 
 ilar to this is seen in the last upper molars of many of the Ungulata, 
 of M'hich JlenisootJieriitm furnishes an average example. Here the pos- 
 tero-internal cusp is absent, and the t^^'o outer cusps are intimately 
 blended. In the rhinoceros (Fig. 257) the last molar goes even further 
 in this direction by reason of the fusion of the elements and the obliter- 
 ation of the external rib. It is conceivable that some such structure as 
 this preceded the pi'esent one in Toxodon, but until the palteontological 
 evidence of the philogeny of this group is more fully known this is the 
 only explanation which can now be offered to account for their aberrant 
 pattern. The pattern of the lower molars is very like that of Jlenis- 
 cothcriimi — a fact which lends countenance to the above hypothesis. 
 
 Teeth of the True Ungulata. — This order ini'lndes" nearly all 
 the modern and many extinct ungulate animals, and is cons]iicuously dis- 
 tinguislicd from all other hoofed forms liy the interlocking character of 
 the proximal and distal rows of the carpal and tarsal bones. Cope has 
 called it the Diplarihm, in allusion to the double articular surface af- 
 forded by the ankle-bone (astragalus) to the cuboid and navicular below, 
 ' I have elsewhere spoken of their relationship to the Tillodontia. 
 
TEETH OF THE VEBTEBBATA. 
 
 477 
 
 whereas in the Condylartlwa the astragalus articulates distally with the 
 navicular or scaphoid only — a condition which obtains in nearly all 
 Mammalia. Two prominent divisions of this order can be recognized 
 — the Artiodactyla, or " split hoofs," of which the hog, cow, and deer, 
 etc. are familiar examples, and the Penssodadijlu, whose only living 
 representatives are the horse, tapir, and rhinoceros. 
 
 The latter sub-order is divisible into a number of sections, which, 
 when we consider the extinct forms constituting at least nine-tenths of 
 the species, we are not able to separate by any characters of very great 
 anatomical importance, notwithstanding the tact that the extremes of 
 the several stems are different enough. That family which stands 
 nearest to the Condylarthra is the Lophiodontidce, a group of extinct 
 generalized perissodactyls from the Middle and Lower Eocene beds. 
 The digital formula is not so great as in the Condylarth:a, being only 
 4 — 3, and in one instance 3 — 3 ; that is to say, four toes on the anterior 
 and three on the posterior limbs.' 
 
 Hyracotherium (Fig. 256) is a typical example of this family, or at 
 
 Fig. 256. 
 
 Skull of Byracotherium augustidens, Cope, from the Wind Elver Beds of Wyoming (after Cope). 
 
 least that section of it whose dentition approaches nearest to Fhenacodus ; 
 and if it were not known that the carpal and tarsal articulations were 
 different, they might easily be mistaken for the same family, so great is 
 the resemblance of their teeth. The dental formula of this animal is 
 I. f , C. 4-, Pm. f, M. f = 44, the same as Phenncodus. The premolars 
 are 'different from the molars, being simpler in form, and the first in 
 both jaws is separated from the others by a diastema. The molar pat- 
 
 iProf. Marsh has described several genera of this group, which he has called Eoliip- 
 pus Orohippus, etc., but the descriptions are so brief that it is impossible to form any 
 correct estimate of their true relationship. Eohippvs, he says, has five toes, but further 
 than this its osteology has not been described. It would be interesting to know in what 
 respects it differs from the phenacodonts, Hyracotherium, Ptiolophus, etc. 
 
478 DENTAL ANATOMY. 
 
 tern is substantially the same as in Phcnacodus, with the slight excep- 
 tion that the cusps are more elevated and laterally flattened, and the 
 external rib is very small or absent in Hyracotherium. In the nearly 
 allied genus Pliolophus, which I suspect to be the same as Orohippus of 
 JNIarsh, the last or fourth premolar below is like the true molars in form, 
 and is quadritubercular, while the genus Lophiotheriuin has the third 
 and fourth premolars below, like the true molars. 
 
 In the second section of this family the external lobes of the superior 
 molars are laterally flattened and intimately blended together, so as not 
 to be well distinguished. Of these the anterior is much the smaller, and 
 is convex externally, ^\•hereas the posterior is large and concave without. 
 The intermediate tubercles no longer exist as such, but form prominent 
 crests which connect the external with the internal cusps, crossing the 
 crown s(jinewliat obliquely. In the lower molars the external and inter- 
 nal cusps are also connected by crests, giving the t}-pical lophodont pat- 
 tern. .Vs a rule, the premolars are trilobed, and the molar formula is 
 Pm. |-, j\I. -|, but in one genus (Dilojyhodon), recently described by Prof. 
 Scott, there are only three premolars in the lower ja^v. In another 
 genus, lately described by the same author under the name of Jjr.suta- 
 totherium, the third and fourth upper premolars are like the molars, 
 and are four-lobed. 
 
 The tapirs form another nearly related family (Tajytridce), which no 
 doubt sprang from some member of the preceding group. The incisors 
 and canines are like those of the Lophiodontklce, but the canines in the 
 lower jaw of the living forms are somewhat procumbent. The third and 
 fourth j)remolars in the upper jaw are like the true molars, A\-hich dis- 
 play the four cusps connected by cross-ridges remarkable for their trans- 
 verse direction in contrast with the oblique crests of some of the preced- 
 ing family. The two external loljes are likewise different in their 
 subequal proportions, both being convex externally and well ■ separated 
 from each other. 
 
 The lower premolars except the first are like the molai's. The exter- 
 nal and internal lobes are connected by strong cross-crests, ^^-hicll are 
 as much elevated as the cusps themselves, and there is no ridge crossing 
 from the postero-external to the antero-internal lobe, as in Hyracothe- 
 rium and Phenacodus. 
 
 From this family we pass to the rhinoceros section of the sub-order. 
 In accordance with what the philosophic student of the living forms 
 would be led to anticipate, this section pertains to a later geologic period 
 than the preceding, and not unnaturally would he seek for the connect- 
 ing links between them and that section of the Lophiodontida-, in which 
 the external lobes are flattened. Through the researches of American 
 palasontologists we are now in a position to fully comprehend all the 
 more important steps in the evolution of this group, and I fail to recall 
 in the whole range of vertebrate palseontology an instance in which the 
 demands of the evolution hypothesis are more completely satisfied than 
 in the present one. 
 
 The molar formula of the rhinoceros is Pm. A M. |-, the usual num- 
 ber in perissodactyles ; but, as regards the incisors and canines, the great- 
 est variability is to be observed. In the two-horned African species 
 
TEETH OF THE VERTEBBATA. 
 
 479 
 
 neither canines nor incisors exist in the adult animal, they having 
 completely disappeared in the course of development. On the other 
 hand, in the remarkable and interesting Eocene genus Orthocynodon 
 ot bcott and Osborn,i the canines are of normal size and erect in posi- 
 tion as the name implies. The number of incisors has not been defi- 
 nitely determined, owing to the imperfect condition of the single specimen 
 
 Fig. 257. 
 
 Skull of Aphelops Tnegalodus, Cope, an extinct American rhinoceros. 
 
 known, but it certainly had two, and probably three, in the lower jaw, 
 as there is abundance of room between the canine and the second incisor 
 for another tooth. If the number is three in the lower jaw, it would 
 imply a like number in the upper, which would bring it very near to 
 the Lophiodontidoe. From this condition of the dentition, which is very 
 nearly that of the Lophiodontidce, we pass to the genus Amynodon of Prof. 
 Marsh, in which the lower canine is much smaller and procumbent, 
 with the incisors reduced to two pairs in each jaw. Following this 
 
 'See Bnlleiin No. 3 Cnntribntions from E. M. Museum of Geology and Archaeology 
 of Princeton College, May, 1883. 
 
480 DENTAL ANATOMY. 
 
 genus in time comes the Lower Pliocene representative Aceratherium, in 
 which the incisors are two upon eacli side in the upper and one in the 
 lower jaw, with the upper canine ahsent. The Middle Miocene fur- 
 nishes a genus, Ceratorhlnus, in which the incisors are one upon each 
 side above and below, and a canine in the lower jaw on! v. Finally, 
 we have a complete disappearance of both incisors and canines in some 
 species now living. The reduction of the incisors and canines trom 
 Orthocynodon to Coelodonta, a li\'ing species, can be summarized as 
 follows : I. I, C. 1, Orfhocipwdnn ; 1. |, C. \, Amynodon ; I. f, C. \, 
 Aceratherium; I. {, C. f, CcratorJiinun ; I. ^, C. f, Coelodonta. 
 
 Fig. 2.5S. 
 
 Superior Molar Dentition of Rhinoceros; a, anterior; ft, posterior end of series. Tl^e figures 1, 2, 3 
 indicate molars and premolars. 
 
 In the earliest forms the molars are more complex then the pre- 
 molars, but in the later and li\'ing species the premolars are as highly 
 organized as the molars, and like them in form ; this is well shown in 
 the accompanying figure. 
 
 About twelve genera have been described, .seven of which come from 
 the fossil beds of North America. Through Orthocynodon, as was 
 pointed out by Profs. Scott and Osborn, they inosculate with the Lo- 
 phlodontidw, after which they branch into several distinct lines. While 
 the rhinoceroses have perpetuated the type of molar which began with 
 the last section of the lophiodonts, other forms inherited the pattern of 
 the hyracotheroids, and from this j)oint the dentition was gradually spe- 
 cialized, not so much througli subtraction of the number of teeth as 
 addition and complication of the diffei-ent lobes and crests of the 
 crowns of both molars and premolars. The culminating point of 
 this line is found in the living hovsc<. 
 
 The first step beyond Hyracotherium in this series is seen in the 
 Eocene genus Ectociuin of Cope, in which the external rib is better 
 defined, the external cusjjs more crcscentic, and the cusps and oblique 
 ridge of the lower molars are more prominent. In the next geological 
 .stage (Upper Eocene) we meet with tiie family Chalicotheriidce, abun- 
 dantly represented in the Wind River deposits by the genus Lambdothe- 
 rium, likewi.se described by C(ipc. In this form (Fig. 259) the external 
 ciisps of the upper molars are considerably elevated, of a crescentic form, 
 and connected -with an external median rib. The anterior cross-crest 
 still has a tubercular form, while the posterior is crest-like and blended 
 with the postero-internal lobe. The four cusps of the inferior molars 
 are connected by cross-ridges and the oblique cr&st, so as to form two Vs, 
 opening internally. The tooth here represented is the last one, which in 
 many of the perissodactyls has a prominent heel (A). In this animal 
 
TEETH OF THE VERTEBBATA. 
 
 481 
 
 ace 
 
 Upper and Lower Molar Tenth or LambM/urivm. 
 vertical view, natural size: a, superior; 6, last 
 inferior molar. In the upper molar, ae, antero- 
 external ; pe, postero-external ; at, antero-in- 
 ternal; pi, postero-internal or principal cusps 
 respectively ; ;v, external vertical rib ; x, an an- 
 terior cingular cusp; aec~pcc, anterior and pos- 
 terior cross-crests. In the lower molar the 
 principal cusps are lettered the same : *, ante- 
 rior basal lobe; <ii', accessory cusp; h, heel. 
 
 the antero-internal cusp (ai) becomes bifid at its summit, and the ante- 
 rior basal lobe (k) again assumes considerable importance. 
 
 Following the chalieotherioids, and as a ])robable derivative of them, 
 we meet with the palseotheiioid.s, in which the molar pattern makes a 
 considerable advance in complexity 
 over that of the preceding family, 
 and the premolars are now like the 
 molars. AncJiitherhim is a good 
 representative of this group, and 
 is here taken for illustration. This 
 genus is of especial interest, in view 
 of its ancestral relation with the 
 horses ; it is here that we get the 
 first distinctive traces of equine 
 peculiarities, although several gen- 
 era intervene between it and the 
 modern Equidse or horses. The 
 species ^vere numerous, most of them 
 equalling the sheep in size, and had 
 three subequal toes on each foot. The incisors, as in all the preceding 
 genera, are plain incisiform teeth, without the pits or "mark" found in 
 the corresponding teeth of the horses. Well-developed canines are like- 
 wise present. 
 
 The superior molars (Fig. 260) display the same elements as those of 
 Lambdotherium ; the external cusps are very much flattened and cres- 
 centic, having their vertical dimensions considerably augmented. The 
 cross-crests form laminar ridges connected with the two internal cusps at 
 the base, and separated from them above byojaen notches; they reach 
 quite across the face of the crown. The 
 two internal cusps almost equal the 
 external ones in height, but have a 
 more conical form ; they are sepa- 
 rated from each other by a deep 
 fissure or valley opening internally. 
 On the posterior border of the crown 
 the cingulum develops an accessory 
 cusp, which has a tendency to form 
 a cross-crest in this situation and enclose a valley between it and the 
 posterior cross- crest. 
 
 In the molars of the lower jaw the same elevation of the lobes and crests 
 is to be observed ; their pattern is substantially that of Lambdotherium. 
 
 In a later geological ej)och the genus Tlippotherium carries dental 
 modification a step farther toward that of the existing horses. The 
 outer toes are much reduced, the incisors possess the peculiar pits of 
 the horse, the molars are more complicated, and the entire appearance 
 is decidedly equine. A strict comf)arison of the elements of the molars 
 with those of Anchitherium is generally difficult, on account of the thick 
 deposit of cement which fills up the valleys and spaces between them. 
 To obviate this difficulty and bring out more clearly the relationship 
 between them, I have represented in Fig. 261 an unworn molar in 
 
 Vol. I.— 31 
 
 Fig. 260. 
 
 Upper and Lower Molars of Rigbt Side of a 
 ST[>eeies ot Ajichitfierium : 1, upper; 2, lower 
 tooth; o, anterior; 6, posterior border. 
 
482 
 
 DEXTAL ANATOMY. 
 
 which the cementum has been removed. Although the respective pat- 
 terns are very much alike in their general structure, the differences con- 
 sist in this : tlie external cusps of the superior molars are relatively 
 larger, more perfectly crescentic, and strongly inclined inward in Hip- 
 potheriiuii. The anterior cross-crest is better developed and joins the 
 posterior cross-crest, so as to enclose a deep pit or valley between it and 
 the antero-external cusp, which is filled with cement in the natural 
 Fig. 261. 
 
 Fig. 262. 
 ai ai' P} 
 
 ae 
 
 A Superior Mular XuuLli or a species oi Hip- 
 polher/unif witli cementum removed : f, an- 
 terior; 6, posterior; c.internal; <;, external 
 borders. Vertical view, natural size. 
 
 Lower Molar of same. Letters as in Fig. 259. 
 
 statu ; this is called the anterior lake in the ^^-orn tooth. The posterior 
 cro,ss-crest bends around to join the posterior cingular cusp, which, with 
 the postero-exterual cusp, furnishes the boundary of the posterior lake. 
 To these crcjss-crcsts are added a greater or lesser number of vertical 
 folds, which give tlie lifirdci-s of the lakes a crenate api^earance when 
 the crown is much -worn. The internal cusps are relatively small, the 
 posterior being connected ^vith the corresponding cross-crest, the ante- 
 rior is<.)]ated. To all these must be added the increased height and the 
 presence of cementum. 
 
 The lower molars (Fig. 1^62) do not exhibit such marked difference 
 from the Anchithci-iiini type as do those above, but they are neverthe- 
 les.-, more complex in their increased depth, complete isolation of the 
 accessory antero-internal cusp, and the addition of cementum. The grind- 
 ing surface of the teeth resulting from this arrangement of the enamel, 
 dentine, and ct'inent is kept constantly rough by reason of the inecjualities 
 in the rate of \\-ear which the.'^^c substances sustain during mastication. 
 Coincident with this structure of the crown the roots disappear and the 
 tooth grows ciintiuuously — a (.'ondition necessary to compensate for the 
 great waste of the tooth-substance. 
 
 Jjastly, we come to the modern horse, in which digital reduction lias 
 readied the extrinue point in this series, or that furthest remo^•ed from 
 the pentedactyle Coiidi/laiihm. As is well known, the digital formula 
 in this family is 1 — 1 in functional use, with the second and fourth 
 represented l)y the rudimentary metajjodials commonly known as the 
 "splint bones." 
 
 The incisors are peculiar and characteristic, inasmuch as the working- 
 face is interrupted by a dce]i pit caused by the uji^vard growth of the 
 posterior cingulum. Previous to extrusion, the posterior wall of this 
 cavity is incomplete and does not rise so high ;is the anterior.^ After 
 
 ' Ryder, " On the Origin and Homologies of the Incisors of the Horse," Proc. Acad. 
 N<il. Sci., Philada., 187?: 
 
TEETH OF THE VERTEBRATA. 
 
 483 
 
 the tooth has been in use for a little while, however, the face i,s worn 
 down smooth, and the central depression appears bounded by a layer 
 of enamel, between which and the enamel covering the outer surface of 
 
 Fig. 263. 
 
 Skull of Hippotherium seversum, Cope (after Cope). 
 
 the tooth, may be seen the dentine. The incisors are not all cut at the 
 same time, the last appearing at the age of five years, on account of 
 which the central pit disappears through wear sooner in those teeth 
 which are first extruded than those which 
 are cut last. By observing carefully the 
 date of appearance of the various inci- 
 sors, and the consequent difference in time 
 at which the pits are obliterated in the 
 diiferent teeth, veterinarians have estab- 
 lished some very useful rules by which 
 the age of a horse can be approximately 
 told with considerable certainty up to ten 
 or twelve years. 
 
 Canines, or the " bridle teeth," are pre- 
 sent, but they are of smaller size, and some- 
 times disappear in the female. The first 
 premolars in both jaws are normally ab- 
 sent, but there are many cases on record in which they are present. 
 Hippotherium they are normally present and functional. 
 
 The molars present essentially the same pattern as those of the pre- 
 ceding genus, the only diiference of importance being found in the 
 
 aec— 
 
 pee 
 
 ^rolar Tooth of a species of Horse. Let- 
 ters as in the preceding figures. 
 
 In 
 
484 DENTAL ANATOMY. 
 
 enlargeaient of the antero-internal lobe and its connection by a ridge 
 with its corresponding cross-crest. Some species of Hippoiherium show 
 a gradual advance from the conic isolated condition of this element to 
 its enlarged and sub-connected form. 
 
 Thus it is that palaeontology has enabled us to fully comprehend 
 the different steps in the productinn of these complex and specialized 
 organs from the simple bunodont pattern. To say that such evidence 
 is witliout its special Ijearing on the great problem of biology, or that 
 evolution or development has not taken place, is to deny the truth of 
 the assertions herein made. Many intermediate steps between those 
 given could be cited, but time and space have compelled me to limit 
 the examples to the most salient. 
 
 The remaining perissodactyles exhibit different degrees of modification 
 of the bunodont type, none having reached the same stage of perfection 
 as the horse. 
 
 The second sub-order of the ungulates, Artiixlacti/ht, attained its 
 greatest development at a later geological period, and it is probably in 
 the present epoch that the genera and species are the most numerous. 
 A few genera are found in the Lower Eocene, but they are of rare 
 occurrence as compared with the perissodactyles. It is probable that 
 they two came off the condylarthrous stem, but the direct evidence to 
 substantiate this supposition is wanting. They are primarily divisible 
 into two group.s, Bunodontid and Sclenndontia, characterized by the 
 pattern of the molar teeth and the conseipient condition of the posterior 
 termination of the maxillary bones. In the former division, of which 
 the hog is an excellent example, the molars have approximately the 
 same pattern as Phenacodus ; the tooth-line is little curved, and the 
 posterior extremity of the maxillary is applied closely to the pala- 
 tine and pterygoid bones, whereas in the Selenodontin the molar teeth 
 have crescentic cusps, and the posterior borders of the maxillaries are 
 separated by a wide sinus from the palatines and pterygoids. These 
 characters at first appear insignificant and inadequate to estalilish and 
 define two such great groups as the fijregoing ; but when we remember 
 that they express a very important structural modification, and that the 
 two are correlated, we cease to express surprise. 
 
 Of these two divisions, the Bunodontia is the older, and as a conse- 
 quence the more generalized. Their generalized characters are most 
 conspicuously displayed in the increased number of digits, bunodont 
 teeth, absence of horns, non-complexity of the stomach, and separate 
 condition of all the limb Iwncs. In fact, the suilline artiodactyles are as 
 primitive in many respects as the Condt/hirthrrt, but in the arranp-ement 
 of the caq>al and tarsal elements they are specialized and far removed 
 from their primitive ancestrv. 
 
 In the hog the dental formula is I. |, C. {, Pm. f, M. f = 44. The 
 outer pair of incisors are small, and sometimes fall out in old age. The 
 canines are relatively large — disproportionally sd in the mah — and in 
 the upper jaw curve round in such a manner that the point of the crown 
 is directed upward. The enamel of these teeth does not uniformly 
 invest the crown, but is disposed in three bands corresponding with its 
 trihedral form. The canines of the lower jaw are more slender and 
 
TEETH OF THE VEBTEBRATA. 485 
 
 have a normal direction. It is said that castration arrests the excessive 
 development of the tusks of the boar, just as this operation profoundly 
 aifects the growth of the antlers of the deer — a circumstance which at 
 once relegates the cause of this condition to sexual influences. 
 
 The -first premolar has no deciduous predecessor, and disappears soon 
 after the adult stage is reached. The rest of the premolars increase in 
 complexity and size from front to rear, but none of them are quadri- 
 tubercular. The first and second molars are quadrate in section, with 
 four-lobed crowns. The last molar is greatly elongated in an antero- 
 posterior direction, which is occasioned by the possession of an enormous 
 heel, much as in the bears, and its crown, as in the others, besides pre- 
 senting the normal four cusps, has an immense number of subsidiary 
 tubercles, giving to it a decidedly wrinkled appearance. 
 
 In the wart-hogs (Phacochoerus) a very peculiar modification of the 
 molar pattern is to be seen in the last tooth. In the unworn state the 
 crown of this tooth presents about thirty small tubercles, arranged in 
 three rows in a direction longitudinal to the axis of the body, the inter- 
 mediate spaces between them being occupied by cementum. When wear 
 takes place, the summits of these cusps are abraded, leaving as many 
 little dentine islands bordered by enamel; they are strengthened by the 
 addition of cement. 
 
 The canines are of enormous size, devoid of enamel, and grow from 
 persistent pulps ; the superior ones are directed upward at first, piercing 
 the upper lip, and then curve backward toward the eye ; their length is 
 sometimes as much as eight or ten inches. All the molar teeth are gen- 
 erally shed in old age, with the exception of the fourth premolar and 
 the last true molar, so that the molar dentition is practically reduced at 
 this time to four upon each side in both jaws, and is the only case of 
 the kind known in the Mammalia. 
 
 The peccaries constitute another family of this division, and are 
 known from the lowest Miocene, if not from the Upper Eocene depos- 
 its. Their molar dentition is more nearly like that of the C'ondylarthra 
 and primitive perissodactyles than other suillines, lacking, as a rule, the 
 great development of the minor tubercles of the molars of the hog as 
 well as the elongated heel of the tooth. The canines, moreover, are nor- 
 mal in direction, and the great disparity in size between these teeth does 
 not exist in the sexes. The incisors are of the usual pattern, although 
 the outer pair is absent from both jaws in some genera. 
 
 From this family the transition is easy to the earlier forms of the 
 selenodonts, in which the feet were multidactyle ; in one genus, Oreodon, 
 as has been recently shown by Prof W. B. Scott, the anterior limb was 
 provided with the normal number of toes, five. That family, which 
 almost completely bridges the chasm between these divisions, is the 
 extinct AnthracotheridcB, whose remains are abundant in the Miocene 
 strata of Europe, but less so in this country. 
 
 It is somewhat uncertain how many genera should be referred to 
 this family, and by what character or characters it should be defined. 
 Fakeoohcerus, which by common consent is a suilline, has four lobes upon 
 the crowns of the superior molars, which are conic and not connected 
 with an external rib, together with two small intermediate cusps, repre- 
 
486 DENTAL ANATOMY. 
 
 senting the cross-crests very much as in Phenacodus. Cheer opotamus, 
 another genus from the Eocene of France, is altogether intermediate 
 between Falceoohcerus and Ardhmeothcriiim, the typical representative 
 of this family, in the pattern of the superior molars ; the external 
 cusps are somewhat crescentic, but the external rib is rudimentary or 
 absent. In the first molar the anterior of the two intermediate tuber- 
 cles only is present, while in the other t\\-o molars it is very small 
 and insignificant; tlir Uvo internal lobes are conic. 
 
 Following this genus in time come Anthracothcrimn, HyopotaiiiuH^ 
 Ancodm, and others in which the anterior of the tw(j intermediate 
 tubercles is the only one which is present in the upper molars. This 
 character, I am therefore disposed to believe, defines a natural group, 
 and should, in connection Avith the external rib and crescentic form of 
 the external cusps, be the test of limitation of this family. 
 
 Two derivatives of the Eocene Hyopotami, Xiphodou, and Anoplo- 
 therium soon Ijecame specialized in their limb structure, but, strangely 
 enough, disappeared in the Early Miocene. Another line was com- 
 menced contemporaneously with that of the anthracotheroid in the 
 genus Didiobunc, wherein the posterioj- intermediate tubercle only was 
 retained. It continues forward through the genus Calnotherium into 
 the Upper ]\Iiocene (lep(_isits of Sansan, where it gradually faded from 
 existence, leaving no modified descendants. This, it appears to me, 
 constitutes another family, definable by the ab(.)ve character. 
 
 From the Anthnicothcridxi have sprung all the modern artiodaetyles, 
 with the possible exception of the cameloids and the existing suillines, 
 together with other stems which are extinct. Many extinct genera 
 complete tlie connections with the living forms in all the osteological 
 and dental details, which it is scarcely within the scope of the present 
 article to discuss. 
 
 In the production of a perfected double crescentic pattern of the 
 superior molars in this sub-order from the short-crowued semi-buno- 
 dont anthracotheroids, the anterior intermediate tubercle has gradually 
 usurped the function of the true antero-internal cusp, it having been 
 reduced to a small cusp situated internal to the mesial horns of the 
 inner crescents on the inner basal portion of the crown (see Fig. 265).^ 
 
 Specialization of the dental organs of the Schnodordia is seen in the 
 following characters : (1) Formation of double crescents in the superior 
 and inferior molars ; (2) great elevation of the cusps and deposit of a 
 thick layer of eemcntum, filling up the valleys ; (.3) loss of the roots of 
 the molars and premolars, and their groA\i:h from persistent pulps ; (4) 
 reduction of the premolars to three in each jaw ; (5) subtraction of the 
 canines and incisors from the upper jaw ; (6) the reduction in size and 
 approximation of the lower canine to the incisors ; and finally (7), the 
 
 ^ Gaudry places the appearance of this genus in tlie sands of Beaiichamp, which 
 probably corresponds with our Bridger Beds or Upper Eocene. He also fixes the date 
 of appearance of Palreochmncs in Europe in the deposits of Saint-Geraud-le-Puy, Middle 
 Miocene. In this country Hynpotninnn does not appear until the Lower Miocene, 
 whereas PalceoclKsnts probably extends into the Bridger epoch. 
 
 ' For a further knowledge of the fossil forms of these families the reader is referred 
 to the important work of Prof. Albert Gandry, "Les Enchainements du Monde animal 
 dans les Temps geologiques/' in which the more important genera are figured. 
 
TEETH OF THE VEBTEBRATA. 
 
 487 
 
488 DENTAL ANATOMY. 
 
 development of a long diastema in front of the premolars. While 
 the complete assumption of these characters is reached only in the 
 bovine ruminants, others exhibit all the intermediate stages of modi- 
 fication tending in that direction. 
 
 The common Virginia deer (Carincus virgudanus) has been selected 
 as an average example of the higher selenodont dentition ; although 
 in its family (Cervid^e) canines are sometimes found in the upper 
 jaw, there is little or no cementum on the crowns of the molars, and 
 they have well-defined roots. It will therefore be observed that it does 
 not fulfil all the requirements of the most highly specialized selenodonts 
 in its dental organization. The dental formula of this species (Fig. 
 265) is I. f, C. -2-, Pm. f, M. | = 32. The incisors have long 
 spatulate crowns, the median pair being the larger, the outer ones 
 decreasing gradually in size. The canines are smaller than the outer 
 pair of incisors, \\'hich they resemble very much in shape, being applied 
 closely to them. ^Vfter an immense interval follow the premolars, the 
 first two in the lower jaw being comparatively simple, the third four- 
 lobed like the succeeding molars. The molars displa}- t^\•o perfect 
 double crescents, of which the outer are convex externally. The last 
 molar has a filth lobe. In tlie upper jaw the premolars are l^ilobed, 
 the internal being convex internally and enclosing a deep valley Ijet^een 
 it and the external cusp. The true molars have double crescents enclos- 
 ing two valleys. The antero-internal of these crescents is made up of 
 the anterior intermediate tubercle, \\'hich lias become greatly enlarged 
 and developed into a crescentie form, the true antero-internal cusp being- 
 situated internal to and beliind it. The proper evidence to support this 
 determination is to be found by examining the superior molars of Hyopot- 
 amus, Anoplotheriu.in, and Xiphodon, it ^vhich it will be seen that the 
 antero-internal cusp becomes gradually smaller. 
 
 Teeth of the Proboscidea. 
 
 The last order of the ungulate series whose dental organs remain to 
 be noticed is that including the elephants, mastodons, etc. The animals 
 composing this group ai'e the largest of terrestrial mammals, and display 
 many curious modifications of the primitive ungulate type. Probably 
 no part of tlieir organization lias been more profoundly affected in their 
 gradual evolutionary growth than th(.' teeth, and were it not for the fact 
 that abundant evidence is at hand to demonstrate the successive steps in 
 the progressive modification from a more simple type, we would be at 
 a loss to comprehend the manner of production of these most complex 
 of all teeth. 
 
 Two genera of proboscideans are found in the existing faunpe of Asia 
 and Africa, but thesi.' are only the inconsiderable remnant of a once 
 greater and much more widely distributed representation, as is indicated 
 by their fossil remains. During the later Tertiaries proboscideans ^^-ere 
 not unknown in both the nortiiern and southern hemispheres in all the 
 extensive land-areas ; in some parts of the northern hemisphere, where 
 they are now extinct, judging from their Ibssil remains immense herds 
 and droves must have at one time existed. 
 
TEETH OF THE VERTEBRATA. 489 
 
 In the African elephant {Loxodon afrioanus) the dental formula is 
 1. -g-, C. ^, Pm. and M. f . The two incisors are greatly enlarged, 
 implanted iu deep sockets, and grow from persistent puljis. They are 
 preceded by small deciduous teeth, and when first protruded are tipped 
 with ejiamel, which soon wears off. The tooth then consists mainly 
 of dentine covered by a thin layer of cement, the dentine presenting 
 a slightly modified form known as "ivory." This substance, as is well 
 known, is extensively used in the arts and has a fixed commercial value. 
 Although not exclusively confined to the tusks of the elephant, never- 
 theless the chief source of supply of this material is derived from them. 
 Tomes cites an example in which a pair of tusks of this species were 
 exhibited in England that weighed three hundred and twenty-five 
 pounds and measured eight feet six inches in length and tA\enty-two 
 inches in circumterence ; the average weight, however, does not exceed 
 from twenty to fifty pounds. The female of this species has tusks quite 
 as large as the male, but in the Indian species the tusks of the male 
 exceed those of the female in size. 
 
 The molar teeth of the living elephants are very much alike in gen- 
 eral pattern and mode of replacement, which is unique ; the description 
 of one will therefore suffice to convey an intelligent understanding of 
 the entire subject. 
 
 Both existing species have a molar formula of ^, A\'hich are divided 
 into milk molars f , true molars f . There is sometimes, in addition to 
 these, a small rudimentary milk molar in front, which increases the 
 total number to seven upon either side in each jaM'. 
 
 Although the total number of molars is normal or nearly so, they are 
 not all in place nor in existence at the same time. Barring the occa- 
 sional rudimentary one, the first molar in the Indian species cuts the 
 gum at a considerable distance from the front of the jaw about the 
 second week after birth. It is implanted by two fangs, and displays 
 a subcompressed crown bearing four cross- ridges, and is therefore lopho- 
 dont in pattern. The upper tooth corresponding to this one cuts the 
 gum a little earlier, and possesses five cross-crests. These teeth are shed 
 at about the age of tAvo years. 
 
 Before the disappearance of the first two teeth the second molars 
 come into place from behind. They are considerably larger than the 
 first, being on an average two and a half inches in length by one inch 
 in breadth. Their crowns are of similar form, but have the number of 
 cross-ridges increased to eight or nine. They are implanted by two 
 fangs, and are shed before the beginning of the sixth year. 
 
 By the time the second molar has been worn out the third molar, 
 averaging four inches in length by two in breadth, makes its appear- 
 ance. Its crown has from eleven to thirteen cross-plates on its Avorking 
 face, and is also supported by two fangs, of which the posterior is much 
 the larger. It is said to be worn out and shed about the ninth year. 
 
 The teeth so far enumerated are taken to be homologous with the 
 second, third, and fourth milk molars of the ordinary diphyodont den- 
 tition which have in this case failed to develop permanent successors. 
 This conclusion is rendered reasonably certain, as we shall presently see, 
 by the fact that their ancestors had a more or less complete permanent 
 
490 DENTAL ANATOMY. 
 
 premolar system, which underwent progressive subtraction as they 
 approached the modern proboscideans. 
 
 Three teeth which are homologous with the permanent true molars 
 are developed behind these in a similar manner. They increase in size 
 and complexity from before backward : the first, or fourth of the entire 
 serie.-i, bears fifteen or sixteen plates ; the second has from seventeen to 
 twenty plates ; while the last supports from twenty to twenty-five. The 
 first true molar disappears between the twentieth and twenty-fifth years 
 of the animal's life, the second somewhere about the sixtieth, while the 
 last is retained until the termination of the animal's natural existence, 
 which is said to be more than one hundred }-ears. 
 
 The structure of these teeth is complex, and, as we have said on a 
 former page, rescmltles that of some of the hystrieine rodents, such as the 
 capvbara, for example. The cross-ridges near their summits are broken 
 up into a number of conical projections, which, when abrasion first takes 
 place, present so many dentine islands surrounded by a rim of enamel : 
 these are arranged in rows across the face of the crown in the position 
 of the future plate (see Fig. 2ri(j). As Avear goes on these islands unite 
 
 Fig. 266. 
 
 Molar Teeth of Indian Elephant {Ekplins imhcus), alter Tomes : «, anterior ; b, posterior border. 
 
 below, and form transverse lamellse composed of a narrow strip of den- 
 tine surrounded by enamel. Between these much-elongated lamellae, 
 which are all blended together at the base of the crown, a thick deposit 
 of cementum is found ; it also invests the lateral surfaces of the crown 
 and prevents fracture of the cross-plates. 
 
 In the growth of the tooth the anterior plates or crests are first 
 formed, and come into position and use long before the posterior. As 
 a consequence of tliis, the most anterior plates wear out and disappear 
 while the posterior ones are still being formed. This is well sliown in 
 the accompanying figure. As new plates are added from behind, the 
 whole tooth moves forward, which probably exerts some influence in 
 the removal of the tooth in front of it. Finally, before the tooth dis- 
 appears altogether, it presents an oval area of smooth dentine sur- 
 rounded Ijy enamel and cementum. It is then no longer efficient as a 
 grinding organ, and is consequently discarded. 
 
 It will be seen by this arrangement of the three tooth-substances on 
 the working surfiice of tlie crown, and by reason of the varying rate of 
 their wear, the teeth of the two jaws when brought into opposition 
 afford most perfect machinery for the grindinn; up of the coarse herba- 
 <'eous substances upon which the elephant feeds. 
 
 The two genera of existing proboscideans may be readilj^ distinguished 
 
TEETH OF THE VERTEBBATA. 491 
 
 by the character of the plates of the molar teeth. In the African species 
 they are fewer in number on the corresponding teeth than in the Indian, 
 and they have a distinct lozenge-shaped pattern upon cross-section, 
 whereas in the Indian species they present an oval outHne upon cross- 
 section and the enamel border is crenate. In the number and succes- 
 sion of the teeth the Uvo genera are alike. 
 
 The genus Dcinotltcrmm includes a few species whose remains have 
 been found in the Miocene deposits of Euroiie, and A\'hic]i were but 
 little if any inferior to the living proboscideans in bodily proportions. 
 They are the oldest representatives of this order so far discovered, and 
 especial interest attaches to their teeth, inasmuch as their structure fur- 
 nishes a clue to a more perfect understanding of the later and more 
 complex types. 
 
 The premaxillary bones M'ere edentulous, but the front part of the 
 lower jaw was provided with two large decurved tusks. What particu- 
 lar use the animal made of these teeth is difficult to imagine. Tlie 
 molar formula is Pm. f, M. |. The structure of these teeth is not very 
 different from that of the tapir, consisting of a moderately short crown 
 bearing two or three cross-crests. Both the premolars had deciduous 
 predecessors, just as in the diphyodonts generally. These animals, 
 however, were very elephantine in every other feature of their anatomy, 
 and were in all probability provided with a trunk. 
 
 From this condition of the dental organs we pass to the mastodons, 
 in which there is a marked approach to the elephants. In some species 
 there were two tusks in each jaw, but the lower ones were small, and in 
 many cases disappeared early in life. The molars increase in complex- 
 ity and size from before backward, the posterior ones bearing in some 
 species as many as ten cross-cresi s, which were unsupported by a 
 cementum deposit ; in others the cross-ridges are much fewer in num- 
 ber. Many species are known, and when all are considered a complete 
 transition between the comparatively simple lophodont and the extreme 
 lamellate patterns is afforded. Many of them had deciduous teeth, 
 which were vertically succeeded by two, and probably three, permanent 
 premolars. As the elephantine molar pattern was acquired, however, 
 these were gradually lost. 
 
 Altogether, it is impossible for a student of odontography to study 
 carefully the teeth of this order, and not be thoroughly convinced in the 
 end that the complex pattern has gradually, but none the less certainly, 
 arisen from the simpler one. If this, therefore, is true of one series, it 
 must be of all. 
 
 The Amblypoda. 
 
 Another order of hoofed mammals which became extinct at the close 
 of the Eocene Period has been described from the fossil-bearing deposits 
 of this country. They were mostly of gigantic proportions, and exhibit 
 affinities with both the proboscideans and the Perissodactyla. They are 
 most nearly related, however, to the Toxeopoda, with which they were 
 contemporary in the Eocene. 
 
 Nearly all of them have the full complement of incisors, canines, pre- 
 
492 
 
 DENTAL ANATOMY. 
 Fig. 267. 
 
 Skull of Lmolaphodon comutus, 
 
 Cope, a species of amhlypod from the American Eocene (after Cope). 
 
 molars, and molars, and in some the canines were greatly enlarged The 
 molar pattern is of moderate complexity, and shows a considerable 
 
TEETH OF THE VERTEBBATA. 493 
 
 departure from the primitive tritubercular, ancestry. In the lower jaw 
 the molars are lophodont, -while in the upper they have a single cres- 
 cent of moderate perfection. Owing to their near relationship with the 
 Toxeopoda, it is highly probable that their teeth represent an extreme 
 modification of the tritubercular pattern, but of the different steps in 
 their production lack of space j^revents me from speaking here. I must 
 refer the reader to the papers of Profs. Cope and Marsh for a more 
 complete description of the dentition of this order. 
 
 Teeth of the Marsupials. 
 
 I have indicated on a preceding page that this division of the Mam- 
 malia is sharply defined from the monodelphs by the circumstance that 
 no connections are formed between the foetal envelopes and the walls of 
 the uterine cavity during gestation, so that no placenta is developed. 
 They are therefore known as the implacental division of the Eidheria ; 
 they are likewise known as the Didelphiasaid MurHupiaUa. The young 
 are born in an exceedingly helpless and imperfect condition, and are 
 transferred to the pouch or marsupium of the mother, where, by a special 
 arrangement, the nourishment is forced into their months until such time 
 as they are enabled to help themselves. 
 
 In the majority of the lower Verfebrata very little development of 
 the young takes place in the body-cavity of the mother ; the ovum is 
 relatively large, by reason of the addition of an abundant supply of pab- 
 ulum sufficient to nourish the embryo until the later stages of develop- 
 ment are reached. It has been recently ascertained that the monotremes 
 reproduce in the same way ; that is, they lay eggs like birds and rep- 
 tiles, Avhich are hatched in a similar manner. The whole plan of devel- 
 opment moreover, is like that of the bird (mesoblastic) — a condition 
 which would be reasonably suggested by a study of their reproductive 
 system. 
 
 As the monotremes furnish the connecting link between the higher 
 mammal and the reptile, so do the marsupials, as far as reproduction is 
 concerned, afford a transitional stage between the monotremes and the 
 monodelphs. For this reason we would naturally be led to look for 
 primitive and transitional characters in their teeth. Unfortunately, 
 these organs do not in many particulars go beyond the lowest forms of 
 the monodelphs sufficiently to give us any clear insight into the inter- 
 mediate structures and patterns which must have preceded the diphyo- 
 dont monodelph dentition ; still, some of the earliest representatives of 
 mammalian existence which have been referred to in this group possess 
 a greater number of heterodont molar and premolar teeth than any 
 known mammal. 
 
 In the small living marsupial genus Myrmecobkis the dental formula 
 is I. A, C. \, Pm. I, M. f = 54. The incisors are small, subconic teeth, 
 implanted in the premaxillary bones above, and followed by the canines, 
 which have the usual laniary form. The premolars have laterally-com- 
 pressed, unicuspid crowns, and are implanted by two roots. The molars 
 exceed in number those of any other marsupial, reaching the unusual 
 number of six in each jaw. Owing to the imperfect descriptions of 
 
494 DENTAL ANATOMY. 
 
 the crowns of these teeth, and never having seen a specimen myself, I 
 am at present unable to say ju^t what the pattern of the crown is. From 
 the best information at my command I suppose it to be somewhat after 
 the style of a modiiied tuberculo-sectorial. I further do not kno'w 
 whether the succession has been observed, and whether a proper distri- 
 bution of the molars and premolars expressed in the above formula has 
 been made; but, judging from the condition in marsupials generally, I 
 am induced to believe it to be correct. It is so gi-\-en by Owen and 
 Waterhouse. 
 
 .Some fragmentary remains, consisting principally of jaws and isolated 
 teeth, of a numl:)er of small mammals have been discovered from time 
 to time in the Jurassic and Triassic deposits of tliis country, Euroiie, 
 and South Africa, in which the teeth behind the canines reach as high 
 a number as twelve in each lower jaw in some species. These are some- 
 what arbitrarily divided into an equal number of molars and premolars, 
 but whether any or all of them had deciduous predecessors is not 
 known. The reason for this division is that the first six behind the 
 canine are premolariforni in shape, while the others possess a number of 
 sharp cusps. They have been referred to the marsupials and assigned a 
 position near to Jlyrmecobius, but until their osteology is better known 
 this is doubtful. Inasnuich as they are the oldest known mammals, we 
 should anticipate on a priori grounds that they really belong to the 
 monotremes instead of the marsupials. The great number of teeth cer- 
 tainly constitutes an approach to the Reptilia, and if they possessed a 
 complete development of a second set, which is not at all improbable, 
 the transition between reptile and mammal w<3uld be in a measure com- 
 plete as regards the teeth. 
 
 Another strange and remarkable genus, Plagiaida.v, together with a 
 number of allies, comes from these ancient horizons. In this animal 
 the molar pattern is complex for so early a representative of the Mam- 
 malia, and is difficult to understand. In the lower jaw of Plagiaula.v 
 there are seven teeth, of whicli the first is large, curN-ed, and pointed, 
 and is probalily an incisor. This is followed after a considerable sjMC'e 
 by four teeth, all of which, except the first, arc implanted by two roots 
 and iu(/rease gradually in size. Their crowns are terminated superiorly 
 I (v a wedge-shaped crest directed antero-posteriorly, whicli is rendered 
 subserrate by the presence of a number of oblique vertical grooves. 
 Behind these are twn smaller teeth with tubercular crowns, which 
 have been supposed to represent true molars. 
 
 Tlie remaining marsupials ^vhich are really known to be such are 
 divisible into the Polyprotodo)ifia, or those of predaceous habits, having 
 many incisors, and tlie Diproindotrfia, vegetable feeders, having onlv 
 two incisors, in the lower jaw. As far as dental characters go, they all 
 agree in the [lossession of four true molars ; there are no^'er more than 
 three i^reniolars, and the deciduous molars, which are succeeded at a 
 comparatively late period by the last premolars, are reiluced to one in 
 each jaw. This, therefore, turnishcs another example wlierein the defi- 
 nition of a premolar is violated. 
 
 Three families are included in the polyprotodont division, one of 
 which, the opossums, is confined to North and South America, and the 
 
TEETH OF THE VERTEBBATA. 
 
 495 
 
 other two to the continent of Australia. As the common Virginia 
 opossum is a good representative of this division, it is here taken for 
 illustration and description. The dental formula is I. |, C. \, Pm. |, 
 ■■ 50. The incisors (Fig. 268) have a truncate cylindroid pattern^ 
 
 Fig. 268. 
 
 M-t 
 
 Dentition of Virginia Opossum (Didelphis mrginianus) : a, upper : 6, lower jaw. 
 
 implated by single fangs, and differ considerably from the correspond- 
 ing teeth of the carnivores, which they exceed in number by two upon 
 each side in the upper, and by one upon each side in the lower, jaw. The 
 canines have relatively the same size and form as in the dog, and indi- 
 cate clearly the carnivorous habits of their possessor. The premolars 
 are simple premolariform teeth implanted by two roots, the first being 
 smallest and separated from the other two by a diastema. 
 
 The molars of the lower jaw are essentially tuberculo-scctorial in pat- 
 tern, with the external cusp of the anterior triangle largest. The heel is 
 tritubercular and of large size. The molars of the upper jaw are inter- 
 esting, inasmuch as they furnisli a transitional stage in the formation of 
 the W pattern described in the moles, shrews, etc. The first molar has 
 the following structure : The crown is triangular in transverse section, 
 with the apex directed inward, at which is situated the antero-internal 
 cusp or the one corresponding with the single internal tubercle of the 
 tritubercular molar. At the antero-external angle is situated a cusp of 
 moderate dimensions, which in perfectly unworn specimens is more or 
 less blended with the cingulum ; just internal to this, upon close inspec- 
 tion, can usually be seen the rudiment of another cusp, which becomes 
 better defined in the second molar. The exact homologies of these two 
 cusps are not clear, but it seems very probable that the external is of 
 cingular origin, and that the one internal to it is the true homologue of 
 the antero-external cusp of the tritubercular tooth. On the outer edge 
 of the crown, posterior to the two just described, is another cusp, which 
 disappears in the last two molars, but which is well defined in the first 
 and second. This cusp is homologous with the one which terminates 
 the median external part of the W in the molars of the shrew and mole. 
 A little posterior to a line drawn between this last-mentioned cusp and 
 the one most internal is another large well-defined tubercle, from which 
 
496 DENTAL ANATOMY. 
 
 a conspicuous ridge passes outward and backward to the produced pos- 
 tero-external angle of the crown. 
 
 It will thus be seen that all the requisite cusps are present in the first 
 and second molars for the production of the W-structure, and that it 
 -would only require the presence of connecting ridges to complete it. 
 
 ^V. distinctive characteristic of this, as ^\■cll as most other marsupials, 
 is seen in the strong inflection of the angle of the jaw and the vacuities 
 caused l:iy failure of ossification in the posterior part of the palatine bones. 
 
 .Vnothcr family of this group includes the Phascogales, Tasmanian 
 devil, the dog-headed opossum, etc. of the Australian continent and 
 neighboring islands. This family is known to naturalists as the 
 Dasi/urkkv, and is distinguished from the opossums proper {Diddphi- 
 dce) \>y having the incisor formula |-. In the genus Plmscof/ole there 
 are three i)remolars in the upper jaw and two in the lower ; in Dasyurus, 
 or the Tasmanian devil, there are only two premolars in each jaw, which 
 number also obtains in the dog-headed opossum {Thylachnni). 
 
 The pattern of the molar teeth of this latter animal is very much like 
 that of Jlc-toin/.r, consisting in the lower series of a principal cone, to 
 which ai'e added anterior and posterior basal cusjjs. The upper 
 molars are tritubercular, as in that genus, but there is a consider- 
 able cingular ledge external to the two outer cusps. 
 
 The lower molars of the other t\vo genera are very similar to those 
 of the opossum, already described. The pattern of the upper molars 
 of IJasyunin have been alluded to in connection with those of the shrew, 
 and need no further description ; those of Phaseogale are essentially the 
 same. 
 
 The bandicoots, constituting the family PeramcUda, are distinguished 
 by an i4icis(.)r formula |-. The canines are reduced, and placed relatively 
 far back in the dentigerous border of the jaws. The molar and pre- 
 molar formula is the same as in the opossum, and there is a similar- 
 ity of pattern in the corresponding teeth of the two families. 
 
 In the second division, Diprotodonfia, the incisors are reduced to two 
 in the lower jaw ; the canines are always small, and in many cases alto- 
 gether wanting, \\hile the molars are more complex, being better adapted 
 to the mastication of a vegetable diet, upon which they principally teed. 
 
 The kangaroo furnishes a typical example of this group, and is here 
 described. The dental formula of Bennett's wallaby (Habnaturus 
 bennetti) is I. -f, C. ■§-, Pm. ^, ]\I. |- = 28. The three pairs of incisors 
 in the upper jaw (Fig. 269) are sul>oqual and closely approximated, 
 except in the middle line, where those of the oj)])osite side are separated 
 from each other by a considerable space. They have incisiform crowns, 
 and are implanted l)y enlarged roots caused by an unusually thick coat 
 of cement. These are opposed by a single tooth on each side below, 
 whose direction is almost a continuation of the long axis of the jaw, 
 so procumbent is its implantation. Tln'v are long teeth ^-ith enamel- 
 covered crowns, slightly compressed from side to side, so as to ]>resent 
 cutting edges on the surfaces which would correspond to the anterior 
 and ]30sterior faces if the tooth were erect, but \Nhich in its present 
 position are superior and inferior. The superior edge bites against, 
 the three upper incisors, opposing them exactly. 
 
TEETH OF THE VERTEBBATA. 
 
 497 
 
 After a long interval come the premolars, which have approximately 
 the same structure in the two jaws as do the molars behind them. The 
 premolars are implanted by two roots, and have crowns whose longitu- 
 dinal diameter greatly exceeds the transverse. The summit of the crown 
 terminates in an antero-posterior ridge, which is bordered at the base 
 
 Fia. 269. 
 
 Dental Series of Kangaroo {Halmaturus benneiti) : a, upper, 6, lower jaw. 
 
 internally in the upper ones by a well-marked cingulum bearing several 
 small cusps ; this cingulum is absent from the inferior teeth. 
 
 The crowns of the molars are highly lophodont, consisting of t-^vo 
 strong transverse crests connected in the median line by an antero-pos- 
 terior ridge. They are all nearly equal in size and alike in both jaws. 
 
 In the phalangers, which constitute another family of this division, 
 the incisors are the same as in the kangaroo. Small canines ai'e usually 
 present, and the premolars may be increased to three in the upper jaw. 
 The third premolar has substantially the same structure as that of the 
 kangaroos, but the molar pattern is selenodont, resembling in this respect 
 the artiodactyle ungulates. They are quadritubercular, the four cusps 
 being crescentic in section, with the crescents reversed in the lower jaw, 
 just as in the artiodactyles. 
 
 Still another family is represented by the wombat, whose dentition 
 exhibits a modification in the same direction as the rodent monodelphs 
 in the reduction of the incisors to a single pair in each jaw and their 
 growth from persistent pulps. The canines are absent, the premolars 
 are ^, and the molars, as well as incisors and premolars, grow contin- 
 uously during the life of the animal. The molar pattern consists of 
 transverse laminse, greatly elongated and united by cement, much as in 
 capybara, one of the rodents. 
 
 A gigantic extinct marsupial animal (Thylocoled) has been described 
 from the late Tertiary deposits of Australia, whose affinities and prob- 
 able habits have provoked a good deal of discussion among English 
 palaeontologists. In each jaw there is a pair of enlarged, hooked, and 
 pointed teeth in the position of the median incisors ; these are followed 
 in the upper jaw by three small teeth, the posterior of which probably 
 Vol. I.— 32 
 
498 DENTAL ANATOMY. 
 
 represents a canine ; in the lower jaw but a single tooth of this kind 
 exists. Next follows a relatively enormous tooth, corresponding in 
 pattern with the single premolar of the kangaroo, and is therefore tren- 
 chant. Behind these are one small tooth in the upper and two of like 
 nature in the lower jaw. 
 
 From the trenchant nature of the large premolariform teeth. Prof. 
 Owen, its describer, has considered it to have been carnivorous in habit, 
 while Prof. Flowei" concludes, from the enlarged incisors and general 
 resemblance of the enlarged teeth to that of the premolars of the kanga- 
 roos, that it is really affiliated with this group and was a vegetable feeder. 
 
 Other marsupials might be mentioned, but the principal modifications 
 of the dental organs of this group have already been set forth in the 
 types selected. 
 
 The Milk Dentition. 
 
 In the preceding pages we have spoken of the deciduous or milk den- 
 tition of the diphyodont Mammalia so far only as they relate to the 
 permanent set in matters of definition. It now remains to discuss the 
 more important question of their true nature and relationship to the 
 permanent teeth in a philosophic sense. Are they superadded embry- 
 onic structures similar to the amnion and allantois, which subserve a 
 temporary purpose and disappear with approaching maturity, or are 
 they to be homologized with the first set of teeth of the lower Verte- 
 brata ? 
 
 Befiire proceeding to a discussion of these questions, it will first be 
 necessary to give a general statement of the more important features of 
 their anatomy, as well as the principal characters in which they differ 
 from the permanent teeth. 
 
 As regards their development, it must be borne in mind that their 
 enamel organs are originally derived from the lining membrane of the 
 oral cavity, or at least that part of it which immediately covers the axes 
 of the jaws, l>y a dipping do\\n of the epithelium, while the dentine 
 organ is developed from the underlying embryonic tissue. The enamel 
 organs in this ease are said therefore to arise de novo. After a time the 
 enamel organs of the permanent incisors, canines, and premolars appear 
 by a process of budding from the necks of the enamel organs of the 
 deciduous teeth, but that of the first molar in the human subject arises 
 de novo, just as those of the tempoi'ary teeth do from the primitive 
 epithelial layer of the mouth. 
 
 From the neck of the enamel organ of this tooth the enamel organ 
 of the second true molar buds out, while the third is derived from the 
 second in a like manner. Whether this order of development is true 
 of all diphyodont mammals is not known, and is a subject which very 
 much needs further investigation. 
 
 The form of the milk teeth resembles that of the permanent ones 
 which succeed them, as a general rule ; an important exception to this, 
 however, is to be observed in the last milk molar, which in the majoritv 
 of cases is more complex than the permanent tooth which succeeds it. In 
 the ungulates the last milk molar in the lower jaw resembles the last 
 
TEETH OF THE VERTEBBATA. 499 
 
 true molar in having three lobes, while in the upper jaw the last two 
 milk molars have the complex pattern of the jyermanent molars. It is 
 a rule of pretty general application that the last milk molar, and in many 
 instances the last two, are succeeded by teeth of a simpler pattern. 
 They may be well developed and retained in the jaw for a consider- 
 able period, as in the dog, or they may be extremely small, and shed, or 
 rather absorbed, before birth, as in some of the seals. There may be as 
 many as six in each jaw, as in the case of the nine-banded armadillo, or 
 they may be reduced to a single one in each jaw, as in the marsupials. 
 The usual number of milk molars is four in what may be called the 
 typical diphyodont dentition, in which there are forty-four permanent 
 teeth in all. Subtractions from this number are of commQn occurrence 
 by reason of the first milk molar failing to develop a permanent succes- 
 sor or its complete disappearance. This, as we have seen, occurs in the 
 dog and many other animals in which the number of premolars is nor- 
 mal. That this tooth is a persistent milk tooth is suggested by the fact 
 that its enamel organ arises de novo, like those of the milk teeth generally. 
 
 In the monophyodonts one set has been lost, and the question nat- 
 urally suggests itself. Which one is it? The very rudimentary con- 
 dition of the milk teeth in the seals, which reaches an extreme point in 
 the elephant seal, has led Prof. Flower to conclude that the single set 
 of the monophyodonts is homologous with the permanent set of the 
 diphyodonts, the first set having become rudimental and finally disap- 
 peared. He further concludes that the milk dentition generally is some- 
 thing superadded, and cannot therefore be homologized Avith the first 
 set of the lower vertebrates. These conclusions are adopted by many 
 authors. 
 
 In the first place, as regards the homology of the single set of teeth 
 of the monophyodonts, there is much plausibility in Prof Flower's 
 position ; but, upon the whole, our information respecting the exact 
 limits of monophyodontism is too meagre to reach any satisfactory results 
 in a solution of this question. It may yet turn out that many of the 
 Cetacea, in which it is thought to be universal, really have rudimentary 
 deciduous teeth in the early stages of growth, as has been suggested by 
 Tomes. Among the edentates the nine-banded armadillo has already 
 been cited as having two sets of teeth, and it does not seem at all improba- 
 ble that all armadillos will ultimately be found to be diphyodont. 
 
 It should also be remembered that an approach to monophyodontism 
 is made in many diphyodonts ; and in all cases in which there is a par- 
 tial loss of one set there can be little doubt that it is the second which 
 has been subtracted. An example of this is afforded hj the probos- 
 cidean series. In Deinotherium there were two and probably three per- 
 manent premolars ; in some species of mastodons they are reduced in 
 number to two or three ; while in the existing elephants they have com- 
 pletely disappeared. The teeth which remain in the position of the pre- 
 molars in these animals are certainly jjerdntent milk molars. The first 
 premolar of the dog, hippopotamus, and others is a case of the same 
 kind. If monophyodontism has been produced in this way, then the 
 single set which remains is not homologous with the permanent set of 
 the diphyodonts, but combines the two, the molar dentition being made 
 
500 DENTAL ANATOMY. 
 
 up of the true molars and persistent milk molars, with the permanent 
 premolars subtracted. 
 
 With reference to the second conclusion, that the milk dentition is 
 something superadded, Dr. Tomes very justly raises objection on the 
 ground that the history of the development of the permanent teeth 
 interposes a difficulty. He says :^ "The tooth-germ of the milk tooth 
 is first formed, and the tooth-germ of the permanent is derived from a 
 portion (the neck of the enamel germ) of the formative organ of the 
 milk tooth. Again, in most of those animals in Avhich there is an end- 
 less succession of teeth, such as the snake, the ne-wt, or the shark, each 
 successive tooth-germ is derived from a similar part of its predecessor ; 
 the natural inference from which would be that the permanent set, being 
 derived from the other, ^\'&fi the thing added in the dijDhyodouts." 
 
 Aside from the inherent improbability of this hypothesis of super- 
 addition of the milk teeth, if the mammal has been derived from the 
 reptile or batrachian — which is true if evolution is true — it is not at 
 all remarkable, but, on the contrary, quite in keeping with the nature of 
 the ease, that the descendants should have retained some of their ancestral 
 features. In the Batraeliia and Rejjtilia there are many sets of teeth 
 developed during the life of the individual, of which the first arises de 
 novo, and all the succeeding ones are derived from that which precedes 
 it. Altogether, I am disposed to regard the diphyodont mammalian 
 dentition in the same light : those teeth which take their origin pri- 
 marily from the epithelial lining of the mouth are strictly homologous 
 with the first set of the lower vertebrates. This would include in the 
 first set the deciduous incisors, canines, molars, and the first true or 
 permanent molars. The second set of the batracliian and reptile would 
 be represented by the permanent incisors, canines, premolars, and sec- 
 ond true molar. The third succession would be represented by the last 
 molar of the diphyodont dentition. 
 
 This view, of course, is leased upon the presumption that the devel- 
 opment of the true molars is the same in all diphyodonts as it is in 
 the human subject — viz. that the enamel germ of the first is derived 
 from the epithelial lining of the mouth ; that that of the second is 
 derived from the neck of the first; and that of the third from the 
 second. 
 
 If it shall be found, however, on further investigation, that in any 
 diphyodont the enamel germs of all the molars arise de novo, then they 
 must in all such cases be added to the first set. This objection may be 
 urged against the view that there are three, or even t-wo, successions 
 represented in the molars of the diphyodont — viz. that they do not suc- 
 ceed each other vertically, as in the case of the reptile and batrachian ; 
 bu*^ this I do not consider of vital importance. There is one thing upon 
 wliicli I would strongly insist, and that is that the first true molar in 
 the human dentition is a persistent milk tooth. 
 
 ' Manual of Dental Anatomy, p. 302. 
 
TEETH OF THE VERTEBRATA. 501 
 
 CONCLUSIONS, ACKNOWLEDGMENTS, ETC. 
 
 Throughout tlie foregoing pages I have endeavored not only to 
 give the leading characteristics of the principal modifications of the 
 dental organs of the Vertebrata, but have in many cases, so far as our 
 knowledge of the extinct forms would permit, endeavored to trace the 
 leading steps in the production of the complex from the simple form. 
 In so doing I have been made aware of the difficulties which beset such 
 an undertaking : the principal burden of these difficulties lies in the com- 
 paratively imperfect knoM-ledge we possess of the palseontological history 
 of certain groups. In others the ancestry is more clearly indicated, and in 
 my judgment the evidence is sufficient to demonstrate with a reasonable 
 degree of certainty the more important steps in their dental evolution. 
 
 The modification of an organ from a simple to a complex structure 
 necessarily implies a cause or force adequate to the production of such 
 result. What, then, is the nature of the force or forces involved, and 
 what is their method of operation ? To simply say that this or that is 
 so, that this tooth is simple and that is complex, without giving any 
 reason why it is so, conveys little information. If one tooth is sim- 
 ple and another complex, there are reasons for it, and it is not only 
 within the province, but is clearly the duty, of the odontologist to dis- 
 cover and point out these reasons if they can be found to exist. 
 
 Two explanations for all such phenomena have been offered. One 
 of these presumes that they were created so by supernatural forces, but 
 as to the nature of these forces we are not informed ; much less do we 
 know about the manner in which it was done. The other assumes that 
 the natural or physical forces, operating through distinct and M-ell-known 
 physiological laws, are alone responsible for the resulting modifications. 
 
 Between these two explanations the naturalist experiences little diffi- 
 culty in deciding which is most in accordance with the observed facts 
 at his command. While the one rests solely upon the vaguest assump- 
 tion, unsupported by so much as a single fact, the other rests upon 
 observed scientific truth, which any one can verify who will take the 
 pains to investigate. When we ascribe these modifications to the physi- 
 cal forces, the conclusion seems inevitable that those of a mechanical 
 nature have been most largely concerned in the modification of form. _ 
 
 The change in form or size of any organ is principally due to addi- 
 tion, subtraction, or transposition of the histological elements of which 
 it is composed ; these, as is well known, are directly dependent on the 
 amount of physiological waste and repair which the organ sustains, or, 
 in other words, the extent of use and disuse. In proportion as an organ 
 or a part of an organ is used, in that proportion will there be increased 
 destruction of its substance and a corresponding determination of the 
 nutritive fluids to supply the loss. The reverse is true of disuse. 
 
 In the harder tissues of the animal body strain and pressure have 
 likewise been potent factors in the determination of form. Recognizing 
 the importance of these influences, Mr. J. A. Ryder has constructed a 
 most ingenious and far-reaching hypothesis in regard to the teeth, which 
 he terms " the mechanical genfesis of tooth-forms." ^ In this he satis- 
 ' Proceedings Acad. Nat. Sciences, Philada., 1878. 
 
502 DENTAL ANATOMY. 
 
 factorily accounts for the forms and patterns of the molar teeth of the 
 ungulates by the manner in which they have used their jaws. He has 
 shown that in the bunodonts the mouth is simply opened and closed 
 during mastication — a movement which is associated with a short- 
 crowned tubercular molar — while in the selenodonts the lower jaw 
 makes an extensive lateral sweep, and is associated with long-crowned 
 crescentic molars. The conclusion is therefore obvious that as the buno- 
 donts were compelled, through force of circumstances, to live upon a 
 diet which required more extensive comminution before it could be 
 properly assimilated, they gradualy develop greater mobility of the 
 lower jaw ; as a consequence of this, the patterns of the molar teeth were 
 modified through pressure in accordance with this movement. If this 
 proposition be true of the teeth of the ungulates, it must likewise be true 
 of all other animals.' 
 
 Dr. Tomes in his Manual of Dental Anatomy criticises Mr. Ryder's 
 conclusions, as follows : " The simple mechanical explanation that the 
 teeth are drawn out into these forms hardly conveys much information, 
 seeing that the tooth, before it is subjected to these influences, is quite 
 finished, and its form, such as it is, is unalterable ; while to effect an alter- 
 ation in the form of a masticating surface an influence must be brought 
 to bear upon the tooth-germs at an exceedingly early period. It might 
 with equal justice be said that the crown of the tooth, being formed 
 thus, had influenced the excursions of the jaw, and so modified the 
 condyle." 
 
 It is evident that Dr. Tomes has either failed to grasp the mean- 
 ing of Ryder's reasoning, or else denies one of the most important 
 principles of the evolution doctrine. I am not aware that Ryder has 
 anywhere asserted that the production of the selenodont pattern of 
 the ungulate molar took place in a single generation, as Dr. Tomes's 
 criticism would seem to imply. As a matter of course, the tooth of a 
 modern ungulate when it comes into position is " quite finished," but 
 were the teeth of the ancestors of the modern ungulates quite finished 
 when they came into position ? Ryder has attempted to show that this 
 finishing process was a gradual one, which took many generations to 
 accomplish, and the facts of palaeontology bear out this view. The 
 bold assertion of Dr. Tomes, to the effect that the masticating surface 
 of a tooth when it comes into position is unalterable, is open to very 
 grave and serious doubts. If the form of a bone or any other organ 
 of the animal body can be influenced by impact and strain, as all evolu- 
 tionists believe, then I can see no reason why a tooth is not amenable 
 to the same influences. 
 
 The suggestion which Dr. Tomes offers, to the effect that the crowns 
 of the teeth have determined the direction of the jaw movements, 
 and so modified the condyle, is somewhat absurd. It is equal to 
 assuming that structure has determined habit — a most remarkable 
 conclusion for an evolutionist of the pronounced type of Dr. Tomes. 
 The fact of the matter is, the evolution hypothesis assumes the very 
 opjiosite of this. I have always believed it to be one of the cardinal 
 
 ^ Dr. C. N. Pierce has elaborated the views of Ryder and made important additions 
 to this mechanical hypothesis. 
 
TEETH OF THE VERTEBBATA. 503 
 
 principles of that great doctrine to consider that structure is largely the 
 result of habit. Upon the whole, I find it quite impossible to harmonize 
 such a suggestion with what this author holds on page 268 of the same 
 work, in which he says : " It would be impossible in these pages to go 
 through the arguments by which Mr. Darwin has established his main 
 propositions ; it must suffice to say here that he has fully convinced all 
 those who are not in the habit, from the fixity of early impressions, of 
 putting many matters upon another footing than that established by the 
 exercise of reason, that any modification in the structure of a plant or 
 animal which is of benefit to its possessor is capable — nay, is sure — of 
 being transmitted and intensified in successive generations until great 
 and material differences have more or less masked the resemblances to 
 the parent form." 
 
 As a result of palseontological investigation we know that the form 
 of the mandibular condyles has been very little, if any, modified, while 
 the teeth have. We know, moreover, that it was a gradual process, and 
 that all complex patterns had their origin in simple ones. 
 
 I feel well satisfied that there is not a single dentition of a compkx 
 nature that has not been profoundly modified by these same mechanical 
 influences. If evolution has taken place as a result of the physical 
 forces, it is impossible to discover any forces sufficient to produce such 
 results other than those of strain, impact, and pressure. These have in 
 some instances probably been exerted upon the young and growing tooth- 
 germs ; in others they have operated upon the adult tooth, thereby fur- 
 nishing the causes for individual variation and determining the direction 
 of the hereditary energies. 
 
 In the preparation of the present article my grateful acknowledgments 
 are due to the following gentlemen : to Prof E. D. Cope of Philadelphia, 
 who has kindly accorded me free access to his large and valuable collec- 
 tion of fossil vertebrates, without which it would have been impossible 
 to include the extinct forms. He has likewise placed at my disposal all 
 the illustrations in his possession which relate to his labors in this field. 
 To Mr. J. A. Ryder for many wise and valuable suggestions in the 
 developmental history of the teeth and other kindred subjects. To 
 Dr. Theo. Gill for the loan of illustrations and much important infor- 
 mation ; and, finally, to Prof C. N. Pierce, at whose instance I was 
 led to undertake the present work. I also wish to express my obliga- 
 tions to this gentleman for much kindly advice and assistance. 
 
 Of the works consulted I have made free use of C. S. Tomes's Manual 
 of Dental Anatomy, a most useful and important work ; also, of the pub- 
 lished writings of Profs. Owen, Huxley, Gegenbaur, Flower, Cope, Leidy, 
 Allen, Eyder, Marsh, and others. 
 
504 DENTAL ANATOMY. 
 
 Descriptions op Plates/ 
 
 Plate I.— Figs. 1 and 2 represent the deciduous incisors and cuspids, with their labial surfaces, 
 and the molars with their buccal surfaces facing. Also the noimal number of roots for these teeth 
 in situ. • 
 
 Figs. 3 and 4 represent the superior incisors, cuspids, and molars with their palatine surfaces, and 
 the full inferior set with their lingual surfaces facing. 
 
 Figs. 5 and 6 represent the mesial surfaces of the full deciduous set, and both mesial and distal 
 surfaces of the molars. 
 
 Plate II. — Figs. 1 and 3 represent the full permanent set of thirty-two teeth, sixteen in each jaw, 
 with the labial surfaces of the incisors and buccal surfaces of bicuspids and molars exposed : a a, the 
 central iucisors, right and left; bh, the laterals; c ^the cuspids or canines; d d, the first bicuspids; 
 e gj the second bicuspids ; fj\ the first molars ; g q, the second molars ; h hj the third molars. 
 
 Figs. 2 and 4 represent the anterior teeth, incisors, and cuspids, with their cutting edges notched, 
 as they are usually seen in the newly-erupted teeth, this uneven or notched appearance usually dis- 
 appearing in a few months, or at most in a year, after eruption. 
 
 Plate III.— Figs. 1 and 2 represent the deciduous or temporary teeth divided longitudinally 
 through their lateral diameter. 
 
 X^'igs. 3 and 4 represent the same teeth divided through their antero-posterior diameters. These 
 cuts give a very accurate idea of the relative size of the crown and roots, and of the position occupied 
 by the pulp-chamber in the same. 
 
 Plate IV. — Fig. 3 gives in contrast -a sectional view of deciduous and permanent upper teeth 
 divided through their lateral diameters. 
 
 Fig. 4, a sectional view of the corresponding lower teeth divided through their antero-posterior 
 diameters, a, b, e, represent, respectively, the deciduous and permanent front incisors in contrast; 
 d, e,f, the lateral incisors ; g, h, i, the cuspids ; Jc, deciduous molars, upper and lower ; and I, m, the suc- 
 cessors to the deciduous molars, the bicuspids; n, o represent permanent molars. c,j\ i, m, o have 
 dotted lines, indicating the thickness of enamel removed by wear, atrophy of the cementum, and reduc- 
 tion in the size of the pulp due to progressive calcification, these changes being incident to old age. 
 
 Plate V. represents in Fig. 1, letters ato h and a to ^ the longitudinal or vertical sections of the 
 sixteen superior teeth, showing the labio-palatine diameter of the pulp-chamber and canal in crown 
 and roots, the section of the molars being through the anterior buccal and palatine roots, while the 
 bicuspids d n and_f^ illustrate the result of such a compression of the fang or root as to divide the 
 puip-chamber into two canals — a condition which so frequently exists in these flattened roots. 
 The double-lettered series, dcE to A A and rfd to hjh, represent in the molars a section through the pos- 
 terior buccal and the palatine roots, from which is quite readily recognized the slightly greater lateral 
 diameter of the pu]p-cbaraber in the crown and the larger canal in the posterior buccal root over that 
 in the anterior buccal root, while the bicuspids lettered t edd and d d e e illustrate a modified pulp- 
 chamber and canal, with bifurcation of the root in one, these being cut through a different axis or plane 
 from the single-lettered series. 
 
 Fig. 2, letters aio h and a_to hj represents the sixteen inferior teeth with the section through their 
 long diameters, as in the superior series. These incisors illustrate the compressed or flattened con- 
 dition of their roots in contrast with the cylindrical character of the roots of the superior iucisors^ 
 while the bicuspids d e and^ illustrate the singleness of their pulp-chamber and the cylindrical con- 
 dition of their roots as in contrast with the flattened or compressed condilion of the roots of the supe- 
 rior bicuspids. The molars /, g, h, and /, g, h represent sections through the anterior root, illustrating 
 its compressed condition aud divided pulp-chamber in the first and second molar, and a somewhat 
 flattened one in the anterior root of the third molar ; //, g g, h /?, and //, g g,h li _ represent the single 
 and cylindrical pulp-chamber in the posterior root of the inferior molars, while b b, c c and aa.b b 
 represent the incisors and cuspids of the same series, with modified pulp-chambers arising from modi- 
 fied development. 
 
 Plate VI. —Fig. 1, from a to A and a to ^, represents the superior teeth, with transverse or horizon- 
 tal section through the base of the pulp-chamber in the crown, viewing the entrance to the canals of 
 the several roots, while the same letters in Fig. 2 represent the inferior series in the same manner. 
 
 Fig. y represents the superior teeth, with the transverse or horizontal section made below the 
 largest diameter of the pulp-chamber and through the canals after they have diverged from the cen- 
 tral chamber, but before the roots into which they run have in the molars bifurcated. 
 
 Fig. 4 in like manner represents the inferior series, well illustrating the flattened or compressed 
 condition of the canal in anterior roots of the molars and the division of the chamber, as is frequently 
 found in the roots of the inferior incisors. 
 
 The letters a a, b b, c c, d d, ff, d_d and ee_ (Fig. 3) represent the relative shapes, whether circular. 
 oval, or flattened, of the pulp-canal in the roots of the superior central and lateral incisors, the cuspids, 
 the flrst and second bicuspids, and the first, second, and third molars, while the same letters in Fig. 
 4 represent the relative shapes of the pulp-canal in similar teeth in the inferior series. 
 
 1 These plates are taken from v. Carabelli's Anatomie des Mundes. 
 
PLATE I. 
 
 For deaeription, see page 504. 
 
 yjjr.y. 
 
 Fu/.^, 
 
 Mf.a. 
 
 ^^.-^. 
 
 ^.^. 
 
 F^.e. 
 
 505 
 
PLATE II. 
 
 For description, see page 604. 
 
 H 
 
 Ns . 
 
 \ 
 
 ^ 
 
 *''^^!!^^**^r% 
 
 •^ > 
 
PLATE III. 
 
 For description, see page 504. 
 
 JF^.J. 
 
 F^.^ 
 
 ^^.3. 
 
 Jf&M 
 
 509 
 
PLATE IV. 
 
 For description, see page 604. 
 
 ^Cl^ 
 
 511 
 
PLATE V. 
 
 For description, see page 504. 
 
 Vol. I.— 33 
 
PLATE VI. 
 
 For description, see page 504. 
 
 % 
 "•55 3 
 
 
 / 
 
 
 
 'S- 
 
 
 
 .■^ 
 
 
 ■1^ 
 
 If 
 
 ^, 
 
 ^ 
 
 U- 
 
 .M 
 
 
 
 m 
 
 ^ 
 
 ^ 
 
 
 / " 
 
 '^lia 
 
 515