XVI INTRODUCTION. 



The first of the orders, the Testudinata, includes the 

 Tortoises and Turtles. Although we have no species 

 of this order inhabiting the British islands, yet as there 

 have been occasionally stray individuals of the marine 

 forms brought to our coasts, and even taken alive, and 

 as many species of the land and fresh- water forms are 

 often kept living in our gardens and ponds, it may not be 

 uninteresting or inappropriate to offer a short account of 

 their general organization and habits. Their structure, as 

 has been already observed, differs in a very remarkable 

 manner from that of the rest of the class. The arrange- 

 ment of the osseous system has already been glanced at ; 

 and it offers the most remarkable tendency to consolida- 

 tion and strength, to the sacrifice of facility and variety of 

 motion. In the terrestrial forms especially this character 

 is carried to an extreme degree ; the vertebra?, the ribs, 

 and the sternum are all closely and inseparably united 

 into a compact solid case, in which the whole of the 

 viscera, and, during rest, the head, limbs, and tail are 

 covered and protected. So strong is this shell, both from 

 the thickness and solidity of its parietes, and from the 

 arched form of the superior portion, that in many species 

 it will bear immense pressure without injury. In certain 

 genera, however, this bony box, although still exceedingly 

 strong, has certain parts which are rendered moveable, for 

 the still more complete protection of the enclosed organs. 

 Thus in the genus Kiniasys, a terrestrial form, the lumbar 

 portion of the carapace, 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 limbs and tail are withdrawn within it, to close that move- 

 able 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. 



