Vlll INTRODUCTION. 



The Reptilia, according to most naturalists, include five 

 orders : the Testudinata, or Tortoises and Turtles ; the 

 Enaliosauria of Conybeare, to which the gigantic fossil 

 genera, the Ichthyosaurus and Plesiosaurus belong ; the 

 Loricata, or Crocodiles and Alligators ; the Sauria, or 

 Lizard tribe, and the Ophidia, or Serpents. 



There is no other class of vertebrated animals, the 

 different groups of which are formed upon types of struc- 

 ture differing 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 by size only, but by 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 organization of the body, — 

 the various systems of which differ only amongst the 

 different groups by comparative degrees of developement. 

 Even amongst the Mammalia, the whale with its enor- 

 mous and almost mountainous bulk, paddled through the 

 deepest retreats of ocean by its short fins, which are 

 modifications of the anterior extremities, and by 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, supported by its thin membranous wings, which 

 are expanded upon slight and linear fingers, the repre- 

 sentatives of the same bones which in the former animal 

 are contracted into a massive and shapeless fin. Nor 

 is there in the other organs of the body any more con- 

 siderable difference of developement. But in the present 

 class, the discrepancies are far more conspicuous, particu- 

 larly in the whole constitution of the skeleton, in the 

 organs of motion, in the integuments, and many other 



