132 



DISCOVERY REPORTS 



Table III (cotitd) 



the pterygoids. The zygomatic arch becomes more massive and the squamosal part of it 

 tends to approach nearer the horizontal, and in the oldest skulls is fused to the maxillary 

 process. The temporal lines approach one another and lie side by side for most of their 

 length in the majority of the oldest skulls, but they are occasionally somewhat separated 

 in the parietal region. Crests for the attachment of the jaw muscles are always more or 

 less developed, but to a degree which is subject to great variation. There is a general 

 increase in the massiveness and rugosity of the skull and a progressive fusion of the 

 cranial sutures. 



Grouping of specimens such as these can only be made by the use of comparative 

 methods ; the descriptions have, therefore, been written principally with reference to 

 conditions occurring in preceding age groups, since this is a series of specimens of 

 increasing age. As I am not concerned with animals before they are sexually active, and 

 have no fresh material from them, reference should be made to the previous paper for 

 data pertinent to the first three annual groups. 



