Vili 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 Sawria, 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 aérial 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 imto 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 
