60 STRUCTURE OF THE VERTEBRATES 



reptiles. They branched directly from the early type, of which 

 the turtles are the only living representatives. Some of these 

 synapsid reptiles carried their legs straighter under the body, 

 not in the typical reptilian position, and the skull assumed 

 mammal-like proportions. Skulls of these orders are illustrated 

 on page 136. 



Order Theromorpha. The Theromorpha were reptiles of more 

 or less ''mammal form". It is the more primitive of the two 

 orders of synapsid reptiles. 



Order Therapsida. The mammal-like characters which showed 

 their beginning in the former order were carried to their logical 

 conclusion in the Therapsida. In size they varied from that of 

 a rat to animals as large as a bear. The legs were held in the 

 mammal position, with the elbows against the body instead of 

 at right angles ; and the skull was almost completely mammalian. 

 By comparison with existing orders of reptiles and mammals, 

 they are much nearer the latter than the former. 



Evolution of the Reptiles. The first reptiles had a body 

 form resembling a shortened lizard and came directly from the 

 amphibian stock. With their adaptations for land life they soon 

 migrated widely and began their own adaptive radiations as 

 the masters of the earth. These early ancestral reptiles and the 

 turtles together form a sub-class, the Anapsida. From this primi- 

 tive stock arose three lines of evolution, giving rise to the three 

 other sub-classes of the Reptilia. 



1. The Synapsida, or mammal-like reptiles, are known from 

 slightly earlier times than the following groups, although it is 

 probable that the three lines arose almost synchronously. 



2. The Parapsida include the snakes and lizards of the present. 

 Several fossil orders are known. The snakes are the most spe- 

 cialized living reptiles. 



3. The Diapsida include the Crocodilia (crocodiles and alli- 

 gators), Rhynchocephalia, and the extinct pterodactyls and 

 dinosaurs. In the great number of forms which have evolved, this 

 is the largest of the reptilian groups, and the dominant animals 

 during the Age of Reptiles. The birds (Class Aves) evolved 

 from a primitive diapsid stock. 



Order Rhynchocephalia. The order is represented today by 



