THE SKELETON IN REPTILES. EXOSKELETON. 271 



ring or joint. New rings are thus periodically added to the 

 base of the rattle, and in old animals the terminal ones wear 

 away and are lost. 



Horny claws occur on the ends of some or all of the digits 

 in most living reptiles. 



Owen's Chameleon bears three epidermal horns, one arising 

 from the nasal and two from the frontal region. 



In the Chelonia, some of the Theromorpha such as Ude- 

 nodon and Dicynodon, probably also in the Pterosauria and 

 Polyonax among the Dinosaurs, the jaws are more or less 

 cased in horny beaks. The horny beaks of Chelonia are 

 variable ; sometimes they have cutting edges, sometimes they 

 are denticulated, sometimes they are adapted for crushing. 



DERMAL EXOSKELETON. 



Nearly all Crocodilia, many Dinosauria, some Rhynchoce- 

 phalia and Pythonomorpha, and some Lacertilia such as Tiliqua, 

 Scincus and Anguis have a dermal exoskeleton of bony scutes, 

 developed below and corresponding in shape to the epidermal 

 scales. Sometimes as in Caiman sclerops, Jacare and Teleo- 

 saurus, the scutes completely invest the body, being so 

 arranged as to form a dorsal and a ventral shield, and a con- 

 tinuous series of rings round the tail. In Crocodilus they are 

 confined to the dorsal surface, and in Alligator to the dorsal 

 and ventral surfaces. The scutes of some extinct forms articu- 

 late with one another by a peg and socket arrangement as in 

 some Ganoid fish. 



The carapace of most Chelonia is a compound structure, 

 being partly endoskeletal and formed from the ribs and verte- 

 brae, partly from plates derived from the dermal exoskeleton. 

 The common arrangement is seen in fig. 36. All the surface 

 plates are probably exoskeletal in origin, but united with the 

 ventral surfaces of the costal and neural plates respectively are 

 the expanded ribs and neural arches of the vertebrae. 



The plastron in the common genus Chelone (fig. 37) 



