DIFFERENTIAL CHARACTERS. 



159 



they giving very nearly the same measurements in respect to 

 extreme dimensions of length and breadth. The difference is 



FIG. 18. Odobamns rosmarus. 



hence one of proportion, resulting from the far greater develop- 

 ment in both breadth and depth of the anterior portion of 

 the skull in the Pacific species. But while the skull of the 



FIG. 19. Odobcenua obcsits. 



Atlantic species is smaller anteriorly than the other, it has the 

 occipital region (Fig. 18) more heavily developed than is the 

 case in the Pacific species (Fig. 19). The difference in develop- 

 ment of the mastoid processes is strongly apparent, not only as 

 respects massiveness, but in the general outline. In the Pacific 



