916 



TEETH. 



crowns of the molar teeth are conical, sub- 

 compressed longitudinally and finely grooved, 

 with an anterior and posterior edge ; those 

 below have generally a slight notch at the 

 fore and back part of the base. The first 

 molars, both above and below, are the smallest, 

 with a simple crown and a single ventricose 

 fang ; the second and third above, and the 

 second, third, and fourth below, have two 

 connate roots ; the two roots are commonly 

 distinct in the remaining posterior molars : all 

 the roots are very thick. 



In the genus Otaria the dental formula is, 



* c. !, p. 



J 



. c. , . , m. = 36. The 



22' 1 1 J 33 22 



two middle upper incisors are small, sub-com- 

 pressed, with the crown transversely notched ; 

 the simple crowns of the four incisors below 

 fit into these notches : the outer incisors above 

 are much larger, with a long pointed conical 

 crown, like a small canine. The true canine 

 is twice as large as the adjoining incisor, and 

 is rather less recurved. The molars have 

 each a single fang ; the crown is conical, sub- 

 compressed, pointed ; in the two last recurved, 

 with a basal ridge or " cingulum," broadest 

 within ; but, in the Otaria jubata, the molars 

 have a pointed cusp developed from the fore- 

 part, and in the last two molars also from the 

 back part of the crown. In some species, as 

 the Otaria lobata (Phoca lobata, Fischer), the 

 single molar is not developed in the upper jaw, 

 and the outer incisors above are not so large : 

 in this species a thick plicated cingulum belts 

 the base of each molar and developes a small tu- 

 bercle from its fore-part in the molars of the 

 lower jaw j the crown of the last molar above 

 is notched. 



In the great proboscidian and hooded 

 Seals (Cystopkora), the incisors and canines 

 still more predominate in size over the molars ; 

 but the incisors are reduced in number, the 



. . 22 11 33 

 formula here is : i. -, r, c. - r, p. ^ - 



1 1 1 1 o o 



22 



m. ~ -- : = 30. All the molars are single- 



<w *^~~ <w 



rooted, and all the incisors are laniariform. 

 The two middle incisors above and the two 

 below are nearly equal ; the outer incisors 

 above are larger. The canines are still more 

 formidable, especially in the males ; the curved 

 root is thick and subquadrate. The crowns 

 of the molar teeth are short, sub-compressed, 

 obtuse ; sometimes terminated by a knob and 

 defined by a constriction or neck from the 

 fang ; the last is the smallest. 



In the Walrus (Trichechus rosmarus), the 

 normal incisive formula is transitorily re- 

 presented in the very young animal, which 

 has three teeth in each intermaxillary bone 

 and two on each side of the fore-part of the 

 lower jaw ; they soon disappear, except the 

 outer pair above, which remain close to the 

 intermaxillary suture, on the inner side of the 

 sockets of the enormous canines, and seem to 

 commence the series of small and simple 

 molars which they resemble in size and form. 

 In the adult there are usually three molars or 



premolars on each side above, behind the per- 

 manent incisor, and four similar teeth on each 

 side of the lower jaw ; the anterior one passing 

 into the interspace between the upper incisor 

 and the first molar, and therefore being the 

 homotype of the molar. In a young walrus's 

 skull with canine tusks eight inches long, I 

 have seen a fourth upper molar, (fifth includ- 

 ing the incisor), of very small size, about a line 

 in breadth, lodged in a shallow fossa of the 

 jaw, behind the three persistent molars. The 

 crowns of these teeth must be almost on a 

 level with the gums in the recent head; they 

 are very obtuse and worn obliquely from above 

 down to the inner border of their base. The 

 molars of the lower jaw are rather narrower 

 from side to side than those above, and are 

 convex or worn upon their outer side. Each 

 molar has a short, thick, simple and solid root. 



The canines are developed only in the 

 upper jaw, but are of enormous size, de- 

 scending and projecting from the mouth, like 

 tusks, slightly inclined outwards and bent 

 backwards; they present an oval transverse 

 section, with a shallow longitudinal groove 

 along the inner side, and one or two narrower 

 longitudinal impressions upon the outer side; 

 the base of the canine is widely open, its 

 growth being uninterrupted. 



The food of the walrus consists of sea- 

 weed and bivalves ; the molars are well 

 adapted to break and crush shells ; and 

 fragments of a species of Mya have been 

 found, with pounded sea-weed, in the stomach. 

 The canine tusks serve as weapons of offence 

 and defence, and to aid the animal in mounting 

 and clambering over blocks of ice. For their 

 composition and microscopic structure I must 

 refer to my " Odontography," p. 51 1. et seq. 



The precise determination of the teeth in 

 the walrus and some other kinds of seals, 

 still awaits the opportunity of examining very 

 young specimens with the deciduous series, 

 which is very early lost. When the clew is 

 afforded by the opportunity of studying the 

 development and succession of the teeth, it 

 infallibly conducts us to the true knowledge 

 of the nature, both of the teeth which are 

 retained, and of those that are wanting to 

 complete the typical number. We have 

 availed ourselves of this in deciphering the 

 much modified dentition of the genus Felis ; 

 and the same clew will guide us to a similar 

 satisfactory knowledge of the nature and 

 homologies of the teeth in the human species. 

 The discovery, by the great poet Gothe, of 

 the limits of the premaxillary bone in man 

 leads to the determination of the incisors, 

 which arc reduced to two on each side of 

 both jaws : the contiguous tooth shows by 

 its shape, as well as position, that it is the 

 canine, and the characters of size and shape 

 have also served to divide the remaining five 

 teeth in each lateral series into two bicuspids 

 and three molars. In this instance, as in the 

 dentition of the bear, the secondary characters 

 conform with the essential ones. But since 

 we have seen of how little value shape or 

 size arc, in the order Carnivora, in the deter- 



