INDIVIDUAL AND SEXUAL VARIATION. 571 



In respect to other features, it may be added that the relative 

 length of the first and second phalanges of the thumb in P. 

 vitulina and P. grcenlandiea is reversed. While the length of 

 the whole digit is nearly the same in the two, the phalanges 

 notably vary, the first phalanx being short and the second long 

 in P. viiuUna, while in P. grcenlandiea the first is long and the 

 second short. The relative length of the digits of the manus is 

 nearly the same in both, P. fcetida, as already stated, being 

 easOy distinguished from either by its having the first digit 

 decidedly the longest and the others successively shorter. 



Individual and Sexual Variation. The wide range of 

 color- variation has been already noted in the general descrip- 

 tion of the external characters, and this appears to be in great 

 measure independent of either sex or age. After allowing for 

 the thickening of the bones and the development of rugosities 

 for the attachment of muscles, there still remains a considerable 

 range of variation in the skull and other bones that may be con- 

 sidered as purely indi\idual. The variations in the skull are 

 shown to some extent by the subjoined table of measurements, 

 but unfortunately very few of the skulls I have examined have 

 been marked for sex. Those known to be those of aged males 

 are noticeably the largest and heaviest, and the most roughened 

 by tuberosities and incipient crests. The largest and heaviest 

 of all is a very old male skull from Santa Barbara Island, Cali- 

 fornia, but this is nearly paralleled by another from the coast of 

 Massachusetts, also that of a very old male. In adult skulls 

 ranging in length from 210 mm. to 223 mm., the greatest width 

 varies from 124 mm. to 144 mm., with corresponding variations 

 in the dimensions of special parts. The nasal bones vary in both 

 length and width fuUy twenty-five per cent, of their mean di- 

 mensions. There is an equally great amount of variation in the 

 width of the muzzle, and nearly as great in the bones of the pal- 

 atal region. The form and size of the narial openings are espe- 

 cially subject to variation, as shown in the subjoined table. 



In both PhoGafcetida and Phoca grcenlandiea the female skull 

 is much smaller, lighter, and weaker in structure than the male 

 skull, and I believe that corresponding sexual differences in the 

 skull obtain in Phoca vitulina^ if indeed they are not even still 

 more strongly marked. Among old skulls two well-marked 

 forms occur, differing in the one being much less massive, 

 smaller, and every way slighter than the other, with the facial 

 portion of the skull narrower and the teeth smaller, the lower 



