In the genus Otaria the dental formula is, i. :3-3 1-1 3-3 33 m. — = 36. The 2-2' C. P. 3-3 2-2 two middle upper incisors are small, sub-corn 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 Nasal 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 kbata. 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 developer a small tu bercle from its fore-part in the molars of the lower jaw; • the crown of the last molar above is notched.
In the great proboseidian and hooded Seals (Cystophora), the incisors and canines still more predominate in size over the molars ; but the incisors are reduced in number, the 2-2 formula here is : c 1-1' 1-1 3-3 1-1' 3-3' m. 2-2 •3= O. All the molars are single 2-2 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 rosnzarus), the , normal incisive formula is transitorily re presented in the very young animal, which has three teeth in each intermaxillary hone 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 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. 511. 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 Fells ; 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 are 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 are, in the order Carnivora, in the deter.