MUSEUM OF COMPARATIVE ZOOLOGY. 33 



Differences resulting from Age. — In color the young differ from the 

 adult, us in most mammals, in being very much darker, especially pre- 

 vious to the first moulting of the pelage. During the first few months 

 the young of both sexes of the fur seals are black, whilst the old males 

 are more or less brownish- or grayish-black, and the females cinereous. 

 In the hair seals the young are dark reddish-brown, whilst the adult are 

 pale yellowish- or grayish-brown. The first coat of hair in the young is 

 somewhat different in character from that they have later, in both the 

 fur and hair species. The latter, whilst quite devoid of fur in adult life, 

 or possessing only an exceedingly sparse undercoat of crisp curled hair 

 rather than fur, are said to have more or less "fur" when young. This 

 is affirmed more especially of the Zalophus hiatus, but doubtless the 

 young of all the hair seals have a softer coat than the adult. 



In respect to the form of the skull, the young greatly differ from the 

 adult, as is sufficiently indicated by the figures of the young and adult 

 skulls of Callorhinus ursinus given in Plates II and III, and described 

 in detail in the account of that species, and as is also shown in the figures 

 of young and adult skulls of Zalophus Gillespii given in the Fauna Japon- 

 ica (Mamm., Plate XXII). It appears that the brain-case early reaches 

 its full size, and changes later mainly through the thickening of its walls. 

 The facial portion is more slowly developed, so that the proportions of 

 the very young and the mature skull are widely different. As regards 

 the general skeleton, my material does not allow me to speak. 



Individual Variation. — In order to determine what characters may 

 be most useful in distinguishing genera and species, it is necessary to 

 take into account the individual variation to which the different parts 

 are subject, as well as the differences resulting from sex and age. For- 

 merly, when but few specimens of any species of the Otariadce were 

 known, it was natural to suppose that any characters based on the adult 

 form of the skull or of its different bones might be regarded as afford- 

 ing reliable specific and generic characters. As more material was 

 acquired, it became evident that these parts in the present group were 

 unusual! v variable, and hence to a great degree unreliable as the foun- 

 dation for specific or even generic diagnoses. The general form of the 

 skull, the depression of the bony palate, the posterior extension of the 

 palatines and their posterior outline, and also the situation of the last" 

 molar relative to the anterior edge of the zygomatic foramen, and the 

 number and form of the molars, have been generally taken as the basis 

 vol. II. 3 



