REPTILIA. CHELONIA. 193 



interclavicle are well developed. In the pelvis, the pubes and 

 ischia meet in a long symphysis. The limbs are pentedactylate, 

 and in the best known forms, the Plesiosauridae, form swim- 

 ming paddles. 



The Sauropterygia occur in beds of Secondary age, and 

 some of the best known genera are Plesiosaurus, Pliosaurus 

 and Nothosaurus. 



Order 3. CHELOXIA. 



In the Tortoises and Turtles the body is enclosed in a bony 

 box, formed of the dorsal carapace, and a flat ventral buckler, 

 the plastron. Except in Dermochelys the carapace is partly 

 formed from the vertebral column and ribs, partly from dermal 

 bones. Both carapace and plastron are, except in Dermo- 

 chelys, Trionyx and their allies, covered with an epidermal 

 exoskeleton of horny plates, which are regularly arranged, 

 though their outlines do not coincide with those of the under- 

 lying bones. The thoracic vertebrae have no transverse pro- 

 cesses, and are quite immovably fixed, but the cervical and 

 caudal vertebrae are very freely movable. There are no lumbar 

 vertebrae. The skull is extremely solid, and frequently has a 

 very complete false roof. Teeth have been detected in embryos 

 of Trionyx but with this exception the jaws are toothless, 

 and are encased in horny beaks. The quadrate is firmly 

 fixed. The facial part of the skull is very short, and the 

 alisphenoidal and orbitosphenoidal regions are unossified. In 

 living forms there are no separate nasal bones, while large 

 pref rentals and postfrontals are developed. There is a com- 

 paratively complete bony palate chiefly formed of the palatines 

 and pterygoids. The anterior nares are united and placed at 

 the anterior end of the skull, and the premaxillae are very 

 small. There is no transpalatine bone and the vomer is 

 unpaired. The dentaries are generally fused together. 



There are ten pairs of ribs, and each rib has only a single 

 R. 13 



