ir. VERTEBliATA: EEPTILIA, CHELONIA. 



595 



garded as an ei^isterniim. It is not connected with the internal 

 skeleton, since the sternum is lacking. The pelvis is only rarely 

 fused with the plastron. This bony caise is usually covered with 

 horny shields, their number and arrangement usually agreeing 

 with the plates of the case, although without their contours exactly 

 coinciding. 



More important are the greai firmness of the skull and the 

 immovable condition of the quadiato, the lack of an os transver- 

 sum and of any but basisphenoid of the sphenoidal bones, and by 



Fig. 623.— Carapace {A) and Plastron (73) nf Tcstudfi graven. (From Wiedersheim.) C, 

 costal plates; t), entoplastron : /!.'/;. epiplastrr)n: //.posterior: H/;, hypoplastron; 

 Hy, hy^)plastron ; M, marginal plates ; A', neural plates ; Np^ nuchal plate ; Pi/, 

 pygal plate : ii, ribs ; V, anterior ; X(, xiphisternum. 



growth forward, and backwards by which the girdles are brought 

 inside the ribs. The teeth are entirely lost, and, as in birds, the 

 jaws are enclosed in sharp horny beaks, in many cases efficient 

 weapons against larger vertebrates. The cloacal opening is oval, 

 its major axis corresponding to that of the body, and in its anterior 

 end is an irnimired erectile penis used in copulation. Turtles 

 appeared in the Permian, and the group has persisted until now. 



Characters of armor and legs serve to contrast sharply the land and 

 sea turtles; the first with well-developed legs, five-toed in front, four- 

 toed behind, the toes with claws; the carapace arched, into which legs, 

 head, and tail may be retracted. In the sea turtles the feet are flipper- 

 like (fig. 634), claws mostly absent, and the carapace weakly united to or 

 free from the plastron, flat and incapable of covering head or appendages. 

 The fresh-water species are intermediate in position. 



Sub Order I. ATHECA. Carapace of numerous mosaic scales and not 

 connected with ribs and vertebrae; skin leathery. Dermochelys iSphargis) 

 coriacea* the leather-back tortoise of warmer seas, reaches a weight of 

 1500 pounds. 



