INTRODUCTION. 



The Reptilia constitute a class of vertebrated animals of 

 which the structural characters are as follow : — They have 

 cold blood, — that is to say, their power of producing animal 

 heat is so limited as scarcely to be appreciable, and not suffi- 

 cient, therefore, to keep up any standard temperature of the 

 body, nor to prevent it from following all the thermal varia- 

 tions of the atmosphere or water by which they are sur- 

 rounded. 



The integument is covered with hard and dry cuticle in 

 various modifications of form, in some constituting broad 

 plates, in others imbricated scales. The heart is in all cases 

 trilocular, — that is to say, it is composed of two auricles and 

 a single ventricle ; the respiration is exclusively pulmonary 

 throughout life, and their reproduction is oviparous. The 

 Amphibia, or hatrachia^ which are included in the Reptilia 

 by Cuvier and many other naturalists, differ from them, how- 

 ever, in various essential and important characters. The heart 

 particularly is bilocular. The integument is naked, and the 

 respiration is carried on by means of branchiae during the 

 earlier period of life, changing in some totally, and in others 

 partially, to the pulmonary character in the adult condition. 



The Reptilia, according to most naturalists, include five 

 orders : the Testudinata, or Tortoises and Turtles ; the Ena- 

 liosauria of Conybeare, to which the gigantic fossil genera, the 

 Ichthyosaurus and Plesiosaurus belong ; the Loricala, or Cro- 



