CHAP. V] THE CRANIA OF THE SIMIIDAE (PRIMATES) 



235 



points. Two of these can be understood best by reference to the 

 accompanying illustration (Fig. 158) in which a tracing (punctuate 

 line) from a young Gorilla skull is superimposed upon a similar 

 tracing (continuous line) from an adult individual. 



1. The basal plate B. PR. elongates with age, and the point B. 

 is brought backwards till in the adult it occupies a new position 

 (B'). This change alone is sufficient to alter the inclination of 

 the plane of the foramen magnum (Op. B.) as shewn in the change 

 produced in the angle Op. B. PR. 



2. The next illustration (Fig. 159) is taken from crania of 

 young and adult examples of the Orang-utan. Here the basal 



Fig. 159 (xj). 



plate of the young individual (punctuate line) has attained its 

 full length. But the hind part of the skull has still to undergo 

 a change in position. This change is partly independent of and 

 accessory to the elongation of the parts between Pr. and B. ; it 

 results in the displacement of the point Op. backwards to a new 

 position (Op'.). This second change is undoubtedly related to the 

 development of the face, i.e. of the jaws and teeth. The amount 

 of this development may be gauged by comparing (in Figs. 158 

 and 159) the facial parts of the skulls concerned, especially in 

 regard to the foremost point (P.) of the upper jaw. 



