DIFFERENTIAL CHARACTEES. 



161 



greatest constriction is considerably i)osterior to the same point 

 in the Atlantic species. 



There are also important differences in the form of the differ- 

 ent bones of the sknll, as shown in the yonng. In the Pacific 

 species (Fig. 22), the nasals are nearly one-third longer and 

 narrower than in the Atlantic species (Fig. 23), and the froutals 

 have a quite different posterior outUne, they being abruptly 



Fig. 21. Odohwmis ohesus. 



narrowed just behind the orbital fossse to less than half the 

 breadth they present in the Atlantic Walrus, and extend further 

 posteriorly in a narrow point instead of being" rather abruptly 

 truncated. In the Atlantic species, the lateral anterior angle 

 of the frontals is in a line with the most laterally projecting 

 portion of the maxiUaries, Av^hile in the Pacific species the 

 breadth at this point is considerably greater than at the ante- 

 rior border of the frontals. While the frontals present in each 

 species a considerable range of variation in resi^ect to their pos- 

 terior outlines, the average difference is very nearly as here rep- 

 Misc. Pub. No. 12 11 



