342 ZOOLOGY sect. 



of the dorsal surface of the tail developed into a longitudinal 

 crest. 



Integument and Exoskeleton. — Characteristic of the Squa- 

 niata is the development of horny plates which cover the eiitire 

 surface, overlapping one another in an imbricating manner. These 

 differ considerably in form and arrangement in different groups ; 

 sometimes they are smooth, sometimes sculptured or keeled. 

 Sometimes they are similar in character over all parts of the 

 surface ; usually there are specially developed scales — the head 

 shields — covering the upper surface of the head. In the majority of 

 Snakes the ventral surface is covered with a row of large 

 transversely elongated scales, the ventral shields. In some Lizards 

 (Chamseleons and Geckos) the scales are reduced and modified into 

 the form of minute tubercles or granules. In some Lizards special 

 developments of the scales occur in the form of large tubercles 

 or spines. Underlying the horny epidermal scales in some Lizards 

 (Skincoids) are series of dermal bony plates. In the integument 

 of the Geckos are numerous minute hard bodies which are inter- 

 mediate in character between cartilage and bone. 



In the Snake-like Amphisbeenians there are no true scales, 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 integument of 

 the Chamseleons 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. Less conspicuous and rapid changes of colour 

 take place in Anguis and in some Snakes. 



In the Ghelonia 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 

 intimately united with elements derived from the endoskeleton 

 that the entire structure is best described in connection with the 

 latter {vide infra). 



In the Crocodilia, the whole surface is covered with horny plates 

 or scales, each usually marked with a pit-like depression about the 

 centre. Underlying each of these, which are of epidermal deriva- 

 tion, is a thick pad of dermal connective-tissue which, in the case 

 of the dorsal scales, is replaced by a bony scute. In the 

 Caimans thin scutes also occur under the ventral scales. 



A periodical ecdysis or casting and renewal of the outer layers 

 of the horny -epidermis takes place in all the Reptilia with 



