INTRODUCTION. XV11 



But it is in the genera Terrapene, Kinostemon, and Sterno- 

 thcenus, 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 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-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- 



