594: CHORDATA. 



divided into two groups, the Lepidosauria, containing the lizards, 

 snakes and Sphenodon, and the Hydrosauria with turtles and croc- 

 odiles. This, however, ignores the fossil forms. When these are 

 taken into consideration another grouping must be adopted. 



Order I. Theromorpha. 



Extinct reptiles from the Permian and triassic which are closely re- 

 lated to the stegocephalous amphibia; with amphiccelous vertebrae, im- 

 movable quadrate, and from two to six sacral vertebras. The ANOMODON- 

 TLA, with partial or complete loss of teeth, stand near the turtles, while 

 the THERIODONTA, in which a heterodont dentition is developed, resemble 

 in this and some other respects the mammals, which, by many, are sup- 

 posed to have descended from them. 



Order II. Plesiosauria. 



Extinct aquatic forms from the triassic to the cretaceous, some forty 

 feet in length. They had long necks, and the limbs were modified into 

 .swimming paddles recalling the flippers of the whales. The quadrate was 

 immovable, and the jaws, with numerous teeth in sockets, were long. 



Order III. Ichthyosauria. 



These forms resembled the Plesiosaurs in skin, swimming feet, elongate 

 jaws, and quadrate, but had the teeth (sometimes absent) in grooves rather 



' 



FIG. 622. Restoration of Plexiosaur. (After Dames.) 



than in sockets, and short necks. Some species at least were viviparous. 

 Their range in time was like that of the preceding order. 



Order IV. Chelonia (Testudinata). 



The turtles form in external appearance a sharply circumscribed 

 group, with the short and compact body enclosed in a bony case, 

 from which only head, tail, and legs protrude (fig. 623). The 

 case consists of a convex dorsal portion, the carapace and a flat- 

 tened ventral plastron, the two being united in most forms at the 

 margins. Each consists of bony plates, the positions and names 

 of which may be learned from the adjacent cut. It only needs 

 mention that the neural plates are united with the spinous pro- 

 cesses, the costals with the ribs, and that the entoplastron is re- 



