DIFFERENTIAL CHARACTEES. 



167 



mandible is far less massive auteriorly tlian in the Atlantic 

 "Walrus (Figs. 29 and 31), wliile it is much more massive idos- 

 teriorly. There is also considerable difference in the mandibles 

 of the two in other respects. Thus, not only is the mandible 

 of the Pacific Walrus much thicker, both laterally and ver- 

 tically, at the symphysis, but the border of the ramus is widely 

 unlike in the two forms. In the Atlantic Walrus (Fig. 30), the 



30. Odobcenus rosmarus. Adult. 



inferior border of the ramus, from the posterior end of the sym- 

 physis to the front of the jaw, rises by a gradual and nearly 

 uniform curve ; in the Pacific Walrus (Fig. 31), the inferior bor- 

 der scarcely rises at all, the jaw in front being simply bluntly 

 rounded. In respect to the posterior portion of the ramus, the 

 differences consist in the greater breadth of the condylar j)or- 

 tion in the Atlantic species, and the greater thickness of the 



Fig. 31. Odobcenus obesus. Adult. 



coronoid process. These differences are all strongly pro- 

 nounced in even quite young skuUs, this being especially the 

 case with respect to the inferior border of the symphysial por- 

 tion of the jaw (Figs. 32 and 34). Another difference consists 

 in the position of the coronoid process, which in the Pacific 

 Walrus, especially in the young, is central to the axis of the 

 ramus, while in the Atlantic species it rises more from the inner 



