INTRODUCTION. 
Tue 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 pro- 
ducing animal heat is so limited as scarcely to be appre- 
ciable, and not sufficient, therefore, to keep up any 
standard temperature of the body, nor to prevent it from 
following all the thermal variations of the atmosphere or 
water by which they are surrounded. 
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 repro- 
duction is oviparous. The Amphibia, or Batrachia, which 
are included in the Reptilia by Cuvier and many other 
naturalists, differ from them, however, in various essential 
and important characters. The heart particularly is bilo- 
cular: the integument is naked, and the respiration is 
carried on by means of branchiz during the earlier period 
of life, changing —in some totally, and in others parti- 
ally —to the pulmonary character in the adult condi- 
tion. 
