STRUCTURE OF THE VERTEBRATES 



135 



dorso-ventrall}' and very wide. The majority of the skull space 

 was taken by the lateral projections, and the huge mouth. 



In the primitive reptile (Seymouria) the skull had deepened 

 and shortened. At the posterior end there was an otic notch. The 

 dermal roof was unbroken except for the anterior nares and the 

 orbits. This is the type skull of the sub-class Anapsida (without 

 an apse, or arch). The turtle is the living representative of this 

 early group (page 60). 



The next development in the skull was the breaking through 

 of an opening (fenestra) in the roof, through which the temporal 

 muscles of the jaw passed to the upper surface of the dermal 

 bones. The brain case was enlarged, though still occupying a 



Fenestra 



Dentarp' 



Surangular 



Fig. 72. Skull of Alligator, a Diapsid Reptile. The face is greatly elon- 

 gated, and two pairs of fenestrae have developed in the temporal region. 



small part of the total mass of the skull. This sub-class was 

 called the Synapsida, through the mistaken belief that the 

 opening was a fusion of two fenestrae. As mentioned (page 60) 

 the mammal-like reptiles carried this development to its logical 

 conclusion. The dinosaurs and Crocodilia were an offshoot from 

 the early Anapsid stem, and with a few less well known orders 

 form a sub-class, the Diapsida, which have two fenestrae on 

 either side. 



The Synapsida were the ancestors of the mammals. A study 

 of the drawings will show that the fenestra became larger, leav- 

 ing a lateral border of dermal bones, with the jaw muscles gain- 

 ing a more prominent footing on the upper surface of the cal- 

 varium. The brain had enlarged, occupying more room in the 

 skull, and the face had shortened materially. In the Therapsida 



