366 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



body are converted into transversely oblong plates, 

 which, when moved rapidly on one another, give out a 

 rattling or vibrating sound. Lizards may be scaleless, 

 like the chamseleon, which possibly makes up for the 

 absence of the protective covering, such as it is, by 

 the power of so adapting itself to the coloration of 

 the neighbourhood in which it finds itself as to be 

 almost invisible; or the Amphisbsena ; or there may 

 be thin scales, as in the true lizards ; or, as in Cyclodus, 

 bone may be deposited in the dermis, and the bony 

 plates may, as in the skink, unite into a mosaic-like 

 arrangement. 



In the Crocodilia there are scutes as well as scales ; 

 that is to say, the dermis undergoes ossification ; and 

 the separate scutes are covered by an epidermic thick- 

 ening or scale. In a few (as the Caiman), the scales 

 on both the upper and lower surfaces become, respec- 

 tively, so united with their neighbours as to give rise 

 to a dorsal or a ventral shield ; on the long tail the 

 upper and lower ossifications unite to form continuous 

 rings. In most crocodiles, however, the ventral 

 shields are absent, and the dorsal scutes do not unite 

 with one another to form such continuous pieces as 

 can properly be called shields. 



The differences between the horny epidermic 

 hardening^ and the osseous dermal thickenings are 

 best exhibited by the Chelonia, where, as is well 

 known, large continuous pieces, both of shell and bone, 

 are ordinarily exceedingly well developed. The 

 thinner epidermic plates form the so-called tortoise- 

 shell, the thicker dermal bones the plates of the 

 shield, ^or carapace, which enter into close connec- 

 tion with parts of the endoskeleton. 



In Birds, the outer covering is in the form of 

 feathers ; a feather consists of a central quill, shaft, 

 or scapus (Fig. 155; d\ from which 011 either side 

 there are given off flattened branches, or barbs; 



