333 



ANATOMY OF VEETEBRATES. 



265 



the outer incisors above arc 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-com- 

 pressed, obtuse ; sometimes terminated by a knob and defined 

 by a constriction or neck from the fang ; the last is the smallest. 

 In the Walrus (Trichcchus rosmarus, fig. 265) the normal 

 incisive formula is transitorily represented in the very young 



animal, which has three teeth 

 in each premaxillary 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 maxillary 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 such molars on each 

 side, behind the permanent 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. The crowns of these 

 teeth must be almost on a level with the gum in 

 skim and Teeth of 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 upper canines are of enormous size, descending and pro- 

 jecting from the mouth,, like tusks, fig. 265, c, slightly inclined 

 outward and bent backward ; they present an oval transverse 

 section, with a shallow longitudinal groove alon; the inner side, 



o O O 



and one or two narrower longitudinal impressions upon the outer 

 side ; the base of the canine is widely open, its growth being 

 uninterrupted. Their homotype below retains the size and shape 

 of the succeeding molars. 



The food of the Walrus consists of sea-weed and bivalves ; the 

 molars are well adapted to break and crush shells ; and frag- 

 ments of a species of Mi/a 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. 



