DIFFERENTIAL CHARACTERS. 



163 



A comparison of the skulls as seen from below^ (Figs. 26 and 

 27) shows not only the considerably greater contraction of the 

 skull anteriorly, and the greater massiveness and different form 

 of the mastoid processes in the Atlantic Walrus, but other 

 weighty differences. These are especially seen in t;he size and 

 form of the auditory bulla?, and, to a less extent, in the form of 

 the occipital condyles, the form of the glenoid cavity, the orb- 

 ital fossaj, etc. In the Atlantic Walrus (Fig. 26), the auditory 



FIG. 24. Odobccnus obesus. 



FIG. 25. Odobcenus rosmarus. 



bullce are relatively larger than in the other (Fig. 27), more 

 quadrilateral in outline, and rather more swollen. The differ- 

 ences in size and outline are very considerable, the auditory bullae 

 in the Pacific species being, as respects outline, nearly triangular. 

 The inner anterior angle is also strongly developed, being by 

 far the most inwardly salient portion of the bulla?, while in the 

 Atlantic skull it is greatly suppressed. As regards the occipital 

 condyles, they are broader and shorter in the Atlantic species, 

 and less produced anteriorly. The space between them is also 



