440 MANUAL OF ZOOLOGY SECT. 



with the exception of the head shields, but the surface is 

 marked out into annular bands of squarish areas. 



In addition to the modification of the scales, the integu- 

 ment of the chamaeleons is remarkable for the changes of 

 colour which it undergoes, these changes being due to the 

 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 ven- 

 tral surfaces are covered by a system of large horny plates. 

 A series of horny head-shields usually cover the dorsal sur- 

 face 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 intimately united 

 with elements derived from the endoskeleton that the entire 

 structure is best described in connection with the latter 



(P- 443). 



In the Crocodilia, the dorsal surface is covered with longi- 

 tudinal rows of sculptured horny plates, beneath which are 

 bony dermal scutes of corresponding form. The ventral 

 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 takes place in all the Reptilia. 

 Sometimes this occurs in a fragmentary manner ; but in 

 snakes and many lizards the whole comes away as a con- 

 tinuous slough. 



The vertebra are always fully ossified. Only in the 

 geckos and Hatteria are the centra amphiccelous, with 



