INDIVIDUAL AND SEXUAL VARIATION. 573 



straight line, while iu still others the anterior teeth are so im- 

 planted, only the posterior two of the series being more or less 

 oblique. 



The variations attending increase of age are chiefly the grad- 

 ual thickening of all parts of the bony framework of the skull, and 

 in the males the development of all the processes for the attach- 

 ment of muscles, of slight rugosities, an incipient sagittal crest, 

 and a more abrupt outward curvature of the maiidibular rami. It 

 is also noteworthy that the teeth are frequently less crowded in 

 the jaw and less oblique in position iu the adult and old-age 

 stages than during the earlier periods of development. It would 

 seem hardly necessary to note the varying position with age of 

 the ridges bounding the temporal muscles, since such variation 

 is usually seen iu mammals which have these ridges well marked, 

 were it not that a difference in the position of the temporal 

 ridges has been cited by Dr. Gray as a character distinctive of 

 his so-called "Halicyon richardsi" as compared with Phoca vitu- 

 lina.* In very young animals the brain-case is smooth, showing 

 no trace of the temporal ridges ; later they are slightly marked 

 and widely diverge ; as the age of the animal increases these 

 ridges become stronger and less divergent, and in very aged ex- 

 amples nearly or wholly meet along the median line of the skull, 

 forming a low, broad crest, slightly divided along the middle by 

 a shallow furrow, which may or may not widen posteriorly into 

 a small flat triangular space, t 



* Hand-List of Seals, etc., 1874, p. 5. 



t For additional remarks on individual variation in the characters of the 

 akull see Clark, Proc. Zool. Soc. Lond., 1873, pp. 556, 557. 



