58 



BULLETIN OF THE 



quite distinct species. In the anterior portion of the skull the differences 

 are nearly as great as in the posterior portion. In the older skull the 

 ratio of the height of the skull at the base of the second molar to its 

 height at the base of the fourth is as 81 to 100; the corresponding ratio 



Fig. 4.* 



in the younger skull is as 74 to 100. It may be added that the same ratio 

 in Dr. Gray's figure of the skull of Zalophus Gillespii\ is as 70 to 100, 

 showing that the younger skull in this character more resembles the Z. Gil- 

 lespii, — which different writers have spoken of as remarkable for the great 

 declination of the face, — than it does the older skull of the same species. 

 There are also great differences in the relative length and shape of the 

 nasal bones, and in the form of the posterior outline of the intermaxillaries 

 (Figs. 1 and 2). In the younger specimen they extend further back than 

 in the older, further even than the end of the nasals, while in the older the 

 nasals extend beyond the intermaxillaries. 



In respect, to the posterior aspect of the skull (Figs. 2 and 4, Plate I), 

 the differences are no less great. The height of the occipital bone is about 

 fifteen per cent greater in the young skull (Fig. 2, PL I), which would be 

 much increased by age through the further development of the supraoc- 

 cipital crest. The breadth of the occiput above is equal in the two; below 

 it is fifteen per cent greater in the older (Fig. 4, PI. I). 



In the lower surface of the skull (Figs. 1 and 3, Plate I) other consider- 

 able differences are observable, and of such a nature that they cannot be 

 dec! as resulting from age. In the older skull, as previously remarked, 

 the bones are in general much thicker than in the younger; but in re- 



* Fig. 4, skull of No. 2920, same view. 



t l'i'oc. London Zoi 1. Society, lbu'J, PI. LXX. 



