438 MANUAL OF ZOOLOGY SECT. 



presence in the dermis of pigment cells which contract 

 or expand under the influence of the nervous system, 

 in a way that reminds one of the integument of the 

 Cephalopoda. 



In the Chelonia, scales, when developed, are confined to 

 the head and neck, the limbs and the tail, but in all of 

 them, with the exception of the Soft Tortoises, both dorsal 

 and ventral surfaces are covered by a system of large horny 

 plates. A series of horny head-shields usually cover the 

 dorsal surface of the head. Beneath the horny plates of 

 the dorsal and ventral surfaces are the bony carapace and 

 plastron, partly composed of dermal bones, but so in- 

 timately united with elements derived from the endoskeleton 

 that the entire structure is best described in connection with 

 the latter (vide infra, p. 441). 



In the Crocodilia, the dorsal surface is covered with 

 longitudinal rows of sculptured horny plates, beneath which 

 are bony dermal scutes of corresponding form. The ven- 

 tral surface of the body is covered with scales like those of 

 a Lizard. The horny plates of the dorsal surface of the 

 tail are elevated into a longitudinal crest. 



A periodical ecdysis or casting, and renewal, of the outer 

 layers of the horny epidermis occurs in all the Reptilia. 

 Sometimes this takes place in a fragmentary manner ; but 

 in Snakes and many Lizards the whole comes away as a 

 continuous slough. 



The vertebra are always fully ossified. Only in the 

 Geckos and Hatteria are the centra amphicoelous with 

 remnants of the notochord in the inter-central spaces. 

 In most of the others the centra are procoelous, a ball-like 

 convexity on the posterior surface of each centrum pro- 

 jecting into a cup-like concavity on the anterior face of the 

 next. 



