136 



STRUCTURE OF THE VERTEBRATES 



the skull had practically assumed the mammal form. The ventral 

 border of the skull below the fenestra had become a distinct 

 zygomatic arch to which the outer jaw muscles (masseter mus- 

 cles) were attached, while the temporal muscles had their origin 

 on the side of the calvarium. The latter pass through the zygo- 

 matic opening to the jaw. 



Fenestra 



Position of 

 zygoma 



Quadrate 

 ■Articular 



Surangular 



Fenestra 



Squamosal 



Articular 

 urangular 



Angular 



Fig. 73. Evolution of 

 the Mammal Skull. (A) 

 Primitive mammal - lilve 

 reptile (Order Thero- 

 morpha) ; (B) Special- 

 ized mammal-like reptile 

 (Order Therapsida) ; and 

 (C) a mammal (Opos- 

 sum). Observe the en- 

 largement of the lateral 

 fenestra, the growth of 

 the brain case, and the 

 loss of the posterior 

 lower jaw bones. 



•Sqiia7nosal 



