VARIATION WITH AGE. 327 



jaw and the teeth. The attachments for the muscles are corre- 

 spondingly less developed throughout the skeleton. The most 

 striking sexual difference, however, is that of size; the weight 

 of the full-grown females, according to Captain Bryant, being 

 less than ONE-SIXTH that of the full-grown males. This estimate 

 Mr. Elliott has since found to be correct by actual weight of 

 large series of specimens. 



DIFFERENCES EESULTING FROM AGKE. The differences in 

 color between the young and the adult consist, as already stated, 

 in the young of both sexes during the first three or four months 

 of their lives being glossy black, and gradually afterwards 

 acquiring the color characteristic respectively of the adult 

 males and females. In respect to the differences in the skeleton 

 that distinguish the young, I can speak only of the skull, in 

 which the relative development of its different regions differs 

 widely from what is seen in the adult of either sex. The two 

 young skulls before me, said to be from specimens thirty -five 

 days old, are both females, but at this age the sexes probably 

 do not differ in osteological features, especially in those of the 

 skull. In these young specimens the anterior or facial portion 

 of the skull is but little developed in comparison with the size 

 of the brain-case. The muzzle is not only excessively short, 

 but the orbital space is small, and the postorbital cylinder 

 is correlatively reduced almost to zero, the postorbital processes 

 being close to the brain-case. The zygomatic arch is hence 

 very short; the zygomatic foramen is as broad as long, instead 

 of being nearly twice as long as broad,,as in the adult. On the 

 other hand, the brain-case is exceedingly large, the greatest 

 breadth of the skull being at the middle of the brain-case in- 

 stead of at the posterior end of the zygomatic arch. As will 

 be seen by the table of measurements of the skull already 

 given, the brain-case is nearly as large as in the adults, and the 

 bones being thinner, it must have a capacity as great as that 

 of the skulls of the adult males and females, there being, in 

 respect to this point, but slight difference in the sexes. As the 

 young advance in age, the anterior portion of the skull, or that 

 part in advance of the brain-case, greatly elongates, especially 

 the postorbital cylinder, and increases also in breadth, the 

 skull in a great measure losing the triangular form and the 

 narrow pointed muzzle characteristic of the young. The post- 

 orbital processes also greatly change their form as they further 

 develop. 



