Sauropsida: Class Reptilia 



493 



Order Chelonia 



Turtles and Tortoises. Body enclosed within a protective "shell" 

 consisting of a dorsal carapace and a ventral plastron joined in 

 the middle region of the body (Fig. 385). Each part is composed of 

 large plates of bone usually overlaid by equally large horny plates or 

 scales; the patterns of the two sets of plates do not coincide. Vertebrae 

 of trunk immovably fused to median (neural) plates of carapace 

 (exceptions mentioned later); only cervical and caudal regions of 

 vertebral column are movable. Ribs of trunk region immovably fused 

 with corresponding paired plates (costal) of carapace (Fig. 385C). 

 Skull massively and compactly built (Fig. 361) ; quadrate and related 

 bones of upper jaw immovably joined; rami of lower jaw firmly 

 united. No teeth; jaws heavily invested with horn which may be so 

 formed as to provide crushing surfaces, cutting edges, or spines sub- 

 stituting for teeth. No sternum. Gastralia possibly represented by 

 certain paired bones (hyoplastron and hypoplastron) of plastron (Fig. 

 385). Legs well developed and variously modified according to the 

 wholly terrestrial, wholly aquatic, or amphibious habits of the species. 

 Tympanic apparatus of ear well developed. Parietal organ not eye- 

 like. Median urinary bladder present and, in some cases, also a pair 



Fig. 385. Skeletons of turtle, Cisludo (left), and sea turtle, Thalassochelys (righl). 

 (C) Costal plate; (Co, Cor) coracoid; (e) entoplastron or episternum; (Ep) epiplas- 

 tron; (F) fibula; (Fe) femur; (H) humerus; (Hyp) hyoplastron; (Hpp) hypolastron; 

 (.//) ilium; (Js) ischium; (A/) marginal plates; (Na) nuchal plate; (Pb) pubis; 

 (Pro) process of scapula (procoracoid ?) ; (Py) pygal plate; (/?) radius; (Sc, So) 

 scapula; (T) tibia; (U) ulna; (Xp) xiphiplastron. (Courtesy, Zittel: "Handbuch der 

 Palaeontologie," Munich, R. Oldenbourg.) 



