MUSEUM OF COMPARATIVE ZOOLOGY. 3 



The collection of skins and skeletons above mentioned of two 

 of the North Pacific species which has recently been received at the 

 Museum of Comparative Zoology throws much light not only upon 

 these species but also upon several of the others. The investigation 

 of this material has led the writer to an examination of the whole 

 group, the results of which are herewith presented. 



Dr. J. E. Gray and others have recently made known the fact that 

 great differences in the form of the skull in Otaria jitbata result from 

 differences in age. Also the existence of remarkably great sexual 

 difference in size has been long established ; whilst Professor Peters, 

 of Berlin, has recently pointed out extraordinary variations in the den- 

 tition of Zalophus Gillespil. The specimens of Callorhinus ursinm 

 and Eumctopias SteUeri in the Museum of Comparative Zoology show 

 that greater and more radical differences even in the osteological char- 

 acters than those previously known are to be expected in all the species. 

 The two adult male skulls of the Eumctopias SteUeri, for instance, dif- 

 fer from each other so much in form that, if their habitat was not pre- 

 cisely known and the evidence of their co-specific relationship unques- 

 tionable, one might well be excused for regarding them as belonging to 

 distinct species ; and the same is true of the two adult male skulls of Cal- 

 lorhinus ursinus. These specimens also show that some of the characters 

 that have been relied on most frequently as affording generic distinc- 

 tion?, — as the form of the palatal surface of the intermaxillaries and of 

 the hinder edge of the palatal bones, — vary so much, not only with age, 

 but in specimens of the same age, that no given form of these parts 

 can be regarded as affording even reliable specific characters. The great 

 degree of asymmetry, especially in the skull, seen in these animals is 

 sufficient to indicate clearly that an unusually great tendency to indi- 

 vidual variation in these animals is to be naturally expected. Professor 

 Peters has already referred to the presence of a supernumerary molar in 

 one side of the upper jaw in two skulls of cared seals in the Leyden 

 Museum, and another instance of the same abnormality is exhibited by 

 one of the skulls of CaUorhinus ursinus previously referred to. Taken 

 in connection with this tendency to variation, the interesting fact that 

 the number of synonymes pertaining to the several species is in almost 

 exact ratio to the number of specimens that naturalists have had for 

 examination is readily explained. The incidental revision of the genera 

 and species embraced in the present paper is based on these recent 

 developments. 



