122 OSTEOLOGY OF CHELONIAN REPTILES. 



OSTEOLOGY OF CHELONIAN REPTILES- 

 TORTOISES AND TURTLES. 



Those animals to wliich, in the manifold modifications 

 of the organic framework, a portable dwelling or place 

 of refuge has been given, in compensation for inferior 

 powers of lomocotion or other means of escape or defence, 

 have always attracted especial attention; and of them the 

 most remarkable, both for the complex construction of 

 their abode as well as for their comparatively high orga- 

 nization, are the reptiles of the chelonian order. The 

 expanded thoracic-abdominal case, into which, in most 

 chelonians, the head, the tail, and the four extremities 

 can be withdrawn, and in some of the species be there 

 shut up by movable doors closely fitting both the ante- 

 rior and posterior apertures, as e. g. in the box-tortoises 

 {cinosternon^ cistudo\ has been the subject of many -and 

 excellent investigations; and not the least interesting 

 result has been the discovery that this seemingly special 

 and anomalous superaddition to the ordinary vertebrate 

 structure is due, in a great degree, to the modification of 

 form and size, and, in a less degree, to a change of relative 

 position, of ordinary elements of the vertebrate skeleton. 



The natural dwelling-chamber of the chelonia consists 

 chiefly, and in the marine species {chelone) and mud-turtles 

 {trionyx) solely, of the floor and the roof; side-walls of 

 variable extent are added in the fresh-water species {emy- 

 dians) and land-tortoises {testudinians). The whole consists 

 chiefly of osseous "plates" with superincumbent horny 

 plates or "scutes," except in the soft or mud-tortoises 

 {trionyx and sphargis\ in which these latter are wanting. 



Fig. 20 shows the manner in which the head and tail 



