216 VERTEBRATE ZOOLOGY 



primitive than the classic Sphenodon, long thought of as the most 

 pj imitive of living reptiles. Varanops and its more slender relatives 

 represent the quick-running or cursorial adaptation as it appeared 

 within the sub-class Parapsida. 



As examples of the secondarily semi-aquatic adaptive types we may 

 cite several members of the sub-class Anaspida, such as Labidosaurus 

 (Fig. 123, B), Seymouria, (Fig. 123, C), and Diadectes, three types that 

 probably lived much as do our modern frogs and salamanders. As an 

 example of the heavy-bodied, and heavily armored type we may cite 

 two members of the sub-class Synapsida: Edaphosaurus and Dimet- 

 trodon (Fig. 123, D), reptiles strikingly characterized by a riotous 

 growth of dorsal spines. These so-called pelycosaurs evidently rep- 

 resent an end product of a very early line of specialization and have 

 left no descendants. 



The Mammal-Like Reptiles (Cynodonts). Another remarkable 

 group of Permian reptiles which appears to have been purely African 

 in distribution was a group of mammal-like reptiles, called Cynodon- 

 tia, believed by the authorities to have given rise to the line from which 

 the mammals arose. These cynodonts (dog-toothed reptiles) showed 

 many tendencies toward mammalian conditions, chief among which 

 were : heterodont dentition (a specialization of the teeth into incisors, 

 canines and molars), more effective types of limbs for rapid land loco- 

 motion, a tendency for the angulare and articulare bones of the lower 

 jaw to disappear, and a tendency for the skull to become completely, 

 roofed over and for the so-called vacuities to disappear. These cyno- 

 donts were evidently carnivorous types of which Cynognathus (Fig. 

 123, E) is a good example. The head of another cynodont, Scymnog- 

 nathus (Fig. 123, F) shows clearly the dog-like dentition. These 

 reptiles are once more to claim our attention when we come to discuss 

 the question of the origin of mammals. 



There is reason to believe that at least five or six other reptilian 

 orders had representatives in the Permian or Permo-Carboniferous: 

 Chelonia (turtles), plesiosaurs (aquatic reptiles), ichthyosaurs (fish- 

 like reptiles), Squamata (primitive lizards), Rhynchocephalia (beaked 

 reptiles), and Parasuchia (primitive crocodiles). Possibly also the 

 great order of dinosaurs had its beginnings in the Permo-Carbonifer- 

 ous, though as yet there is no direct evidence of their presence during 

 this period. 



A number of orders of reptiles not only had their origin during the 



