COLD-BLOODED VERTEBRATA 



393 



with horny plates and are very effective both for feeding and 

 as weapons, forming, as they do, part of a very rigid skull. 

 The carapace is composed of a number of dermal bones, 

 some of which are fused, to parts of the endoskeleton. In 

 the middle is a row of small neural plates, fused to the 

 neural spines of vertebras. On each side of this row is a 

 set of costal plates, fused to the ribs. A ring of marginal 

 plates, completed in front 

 by a nuchal plate, out- 

 lines the whole, and some 

 little pygal plates fill a 

 gap at the hinder end of 

 the neural series. Since 

 the plates of the carapace 

 are sutured together, the 

 ribs and vertebrae which 

 are fused to them are 

 immovable. The plastron 

 is also composed of 

 dermal bones, some of 

 which represent the clavi- 

 cles and interclavicle of 

 other reptiles. Over the 

 carapace and plastron the 

 scales are represented by 

 large, horny, epidermal 

 scales or shields of " tor- 

 toise-shell," arranged in a 

 regular pattern but not 

 corresponding to the 

 underlying bones. Nerve 



endings give the tortoise-shell a certain sensitiveness. The 

 two halves of the shell are strutted by a characteristic 

 tripod-shaped shoulder girdle, in which the precoracoid 

 has disappeared, its place being taken by a well-developed 

 acromion process. We shall find this structure, small in the 

 frog, well developed in mammals. No doubt the plastron 

 makes a sternum needless. 



Crocodiles, alligators, and gavials (Crocodilia) are lizard- 

 shaped reptiles with bony dermal plates corresponding to 

 the epidermal scales of the back, two arcades and a fixed 

 26 



Fig. 284. — A dorsal view of the cara- 

 pace of a turtle. — From Reynolds, 

 after Owen. 



1, Nuchal plate; 2, first neural plate; 3, 

 second costal plate ; 4, marginal plate ; 

 5, pygal plates ; 6, rib ; S and 9, outlines 

 of first vertebral and third costal epi- 

 dermic shields. 



