526 



CHORDATA 



however, ignores the fossil forms. When these are taken into considera- 

 tion another grouping must be adopted. 



Order I. Theromorpha. 



Extinct reptiles from the Permian and triassic, closely related to the stego- 

 cephala; with amphiccelous vertebrae, immovable quadrate, and from two to 

 six sacral vertebrae. ANOMODONTIA, with partial or complete loss of teeth, 

 near the turtles; the'THERioDONTA, in which a heterodont dentition is devel- 

 oped, resemble mammals, which, by many, are supposed to have descended 

 from them. 



Order II. Plesiosauria. 



Extinct aquatic forms from the mesozoic, some of them forty feet long. 

 They had long necks; limbs were swimming paddles recalling the nippers of 

 whales; quadrate immovable; jaws long, with numerous teeth in sockets. 



Order III. Ichthyosauria. 



Resembled the Plesiosaurs in swimming feet, elongate jaws, and quadrate, 

 but had teeth (sometimes absent) in grooves, and short necks. At least some 

 species were viviparous. Mesozoic. 



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 (figs. 575, 576). The case consists of a convex 

 dorsal portion, the carapace and a flattened ventral plastron, the two being 

 united in most forms at the margins. Each consists of bony plates, the posi- 

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

 mention that the neural plates are united with the spinous processes, the costals 



R 



FIG. 575. Carapace 0-1) and plastron (B) of Testudo arceca (from "Wiedersheim) . 

 (', costal plates; E, entoplastron; Ep, epiplastron; H, posterior; Hp, hypoplastron ; Hy, 

 hyoplastron; M, marginal plates; AT", neural plates; Np, nuchal plate; Py, pygal plate; 

 R, ribs; V, anterior; Xi, xiphisternum. 



with the ribs, and that the entoplastron is regarded as an episternum. It is not 

 connected with the internal skeleton, since the sternum is lacking. The pelvis 

 is only rarely fused with the plastron. This bony case is usually covered with 

 horny shields, their number and arrangement usually agreeing with the bony 

 plates, although without their contours exactly coinciding. 



More important are the great firmness of the skull and the immovable 



