INTRODUCTION. XVll 
But it is in the genera Terrapene, Kinosternon, and Sterno- 
thenus, all of them aquatic or palustrine forms, that this 
peculiarity is the most obvious. In the first-named genus 
especially, the sternum is formed of two distinct valves, 
moveable upon a single cartilaginous hinge, and capable of 
completely closing the bony box, of which it thus forms a 
double lid; within this admirable shelter the animal is 
wholly included, and, if disturbed, resists, by means of its 
powerful muscles, every attempt to open it. The general 
form of the carapace, or upper part of the shell, is also 
admirably adapted to the habits of the different groups of 
which this order is composed. In the land Tortoises it is 
strong, compact, elevated, and regularly arched, for the 
purpose of resisting the numerous injuries to which their 
localities, and especially their extreme slowness of move- 
ment, must otherwise expose them. In the aquatic species, 
on the contrary, it is remarkably flattened, especially in 
the genus 7’rionyx, which has also the margins of the ribs 
free towards their extremities, and the whole body covered 
with a coriaceous skin, which is free at the edges, and 
serves the same purpose as the flattened lateral fins of the 
flat fishes, such as the sole and plaice,—namely, to enable 
them to scuttle themselves under the mud or sand at the 
bottom of the rivers or lakes in which they reside. These 
last have also a very long and retractile neck, by which 
they are able, when thus concealed, to seize fish which 
pass immediately above them, by suddenly stretching out 
the neck, and then as suddenly withdrawing it. The land 
Tortoises, which are exclusively vegetable feeders, are slow 
in their motions, the limbs being clumsy and club-shaped. 
The fresh-water forms are all of them carnivorous; the 
neck is therefore long, the legs lax and flexible, and the 
feet palmated to enable them to pursue their prey with con- 
