Vlll INTRODUCTION. 



codlles and Alligators ; tlie Saun'a, or Lizard tribe, and the 

 Ophidta, or Serpents. 



There is no other class of vertebrated animals the dif- 

 ferent groups of Mhich are formed upon types of structure 

 diiferinof so essentially from each other as these. The 

 eagle and the humming-bird, the ostrich and the. petrel, 

 widely as they appear to be separated from each other, not 

 bv size onfy, but bv form and habits, still exhibit the same 

 general structure of the skeleton, of the organs of digestion 

 and motion, of the integument, and, in fact, of the whole or- 

 ganization of the body, — the various systems of which differ 

 oidy amongst the different groups by comparative degrees of 

 developement. Even amongst the Mammalia, the whale 

 with its enormous and almost mountainous bulk, paddled 

 through the deepest retreats of ocean by its short fins, which 

 are modifications of the anterior extremities, and bv that 

 broad expanded oar, its fleshy tail, — is still formed upon the 

 same general plan of organization, as the little light and aerial 

 bat, which flits so rapidly through the regions of air, sup- 

 ported by its thin membranous wings, which are expanded 

 upon slight and linear fingers, the representatives of the same 

 bones, which in the former animal are contracted into a mas- 

 sive and shapeless fin. Nor is there in the other organs of 

 the body any more considerable difference of developement. 

 But in the present class, the discrepancies are far more con- 

 spicuous, particularly in the whole constitution of the ske- 

 leton, in the organs of motion, in the integuments, and many 

 other important portions of their organization. If, with 

 Cuvier, and most other Zoologists, we include the amphi- 

 bious group in this class, these discrepancies are still more 

 remarkable ; but even restricting our view to the Reptilia 

 proper, they are sufficiently striking ; and a slight glance at 

 the general structure of two of the orders will exhibit them in 

 a very obvious point of view. 



