5L'li 



CIIORDATA 



ho\ve\er, ignores the fossil forms, \\lien these are taken into considera- 

 tion another grouping must be a(.lopted. 



Order I. Theromorpha. 



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

 cephala; with amphica'lous \erlebra\ inimovahle ([uadrate, and from two to 

 six sacral \'ertel)ra-. Anomodon'Tia, wilh partial or complete loss of teeth, 

 near the turdes; the Thkriodonta, in which a Iielcrodont dentition is devel- 

 oped, rcsendilc mammals, which, by many, are supposed to ha\'e descended 

 from them. 



Order II. Plesiosauria. 



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

 Thev had long necks; limbs wire swimming paddles recalling the fli])pers of 

 whales; quadrate immo\able; jaws long, with numerous teeth in sockets. 



Order III. Ichthyosauria. 



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

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

 species were \iviparous. ^Mcsozoic. 



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 wdiich 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 



^y // Ty 



Fig. 575,— Carapace (A) ami plaslron (B) ot Tcshido s^r.rdi if roni \\'ie<lcrs1ieim') . 

 C, costal plates; A", unloplaslroii ; /■'.p. (.-piplastron ; II , posterior; ///>. hv]ioplaslron; // v, 

 hyoplastron; .1/, marginal ]>lalcs; N. neural plates; A'/>, nuchal plate; I'y, pygal jilale; 

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



with the ribs, and that the cntophulron 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 skuil and the immovable 



