XVI INTRODUCTION. 



or upper part of the case, is moveable, so that the animal has 

 the power, when the limbs and tail are withdrawn within it, to 

 close that moveable piece against the posterior part of the 

 sternum ; and in the genus Pyxis the anterior portion of the 

 sternum exhibits this peculiarity in a still more remarkable 

 degree. But it is in the genera Terrapene, Kinosternon, 

 and Sternotheenus, 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 com- 

 pletely closing the bony box, of which it thus forms a double 

 lid ; within this admirable shelter the animal is wholly in- 

 cluded, and, if disturbed, resists, by means of its powerful 

 muscles, every attempt to open it. The general form of the 

 carapax, 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, ele- 

 vated, and regularly arched, for the purpose of resisting the 

 numerous injuries to which their localities, and especially 

 their extreme slowness of movement, must otherwise expose 

 them. In the aquatic species, on the contrary, it is remarka- 

 bly flattened, especially in the genus Trionyx, 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- 



