72 THE VOYAGE OF H.M.S. CHALLENGER. 



regard to the length, thickness, and curvatures of the tusks, I find considerable 

 differences in the series of crania in the Anatomical Museum of the University. As a 

 general rule they diverge from each other so as to be two, three, or even four inches 

 further apart at the tip than at the root, but in one very fine specimen with long and 

 relatively thin tusks projecting 15 inches beyond the alveolus, the distance between their 

 tips was a trifle less than that between their roots ; it may be a question therefore if this 

 character is more than individual or perhaps sexual. As to the front profile and the 

 relation of the transverse plane of the anterior border of the nasals to the base of the tusk, 

 I find that in my specimens this plane sometimes corresponds with the posterior border, 

 at others it extends up to or indeed a little in front of the middle of the base of the tusk, 

 so that it is obviously a variable feature. 



Mr. Allen also refers, though without attaching so much importance as with the other 

 characters, to a difference in the intermaxillaries in these animals. Usually, he says, in 

 the Pacific Walrus the interinaxillse extend posteriorly for two-thirds the length of the 

 nasal, whilst in the Atlantic animal these bones do not enter into the dorsum of the skull, 

 but end at the anterior border of the nasals. In those of my specimens, in which these 

 bones had not yet ankylosed with their neighbours, I noticed considerable variation ; in 

 four each premaxilla articulated with the anterior two-thirds of the outer border of the 

 nasal, in two each premaxilla reached the anterior border of the nasal and then seemed to 

 terminate, but an elongated sutural bone was intercalated on each side between the 

 superior maxilla and about the middle third of the outer edge of the nasal ; in one the 

 left premaxilla articulated with the anterior two-thirds, whilst the right bone only 

 reached the tip of the nasal, but beyond it was a sutural bone similar to that above 

 described. This intercalated bone obviously represents the detached upper end of the 

 premaxilla. It is obvious that a bone presenting such variations in arrangement in the 

 skull of the Walrus as does the premaxilla cannot have much importance attached to it 

 for purposes of classification. 



As regards the external features of difference Mr. Allen states that the two animals 

 are similar in size and probably in general contour, though the facial outline is modified 

 by the differences in the skull already considered, but the mystacial bristles are shorter 

 in the Pacific than in the Atlantic Walrus. 



On the whole I think it is doubtful whether these animals should be regarded as 

 specifically distinct ; I would rather consider them as varieties of one species. 



Otariida 



In no family of mammals, probably, have more diversities of opinion been expressed 

 by zoologists, both with respect to the number of species in the family and their arrange- 

 ment in genera and subfamilies, than in the Otariidse. These divergences are to be seen 



