CHELONLAN REPTILES. 470 



separate from each other, and resume their usual 

 form ; the intervening spaces between two adja- 

 cent ribs being here filled up by membrane. 

 The plastron is united with the carapace by 

 membrane likewise ; and the sternum, instead 

 of forming one broad plate of bone, has the in- 

 tervals between its imperfectly developed ele- 

 ments also membranous. All this renders the 

 whole shell less compact, more flexible, and 

 more feeble : but the movements of the animal 

 are quicker and more energetic. 



These characteristic differences between the 

 aquatic Chelonia and those that live on land are 

 still more strongly marked in the genus TrionyXy 

 or soft tortoise ; which is destitute of scales, and 

 in which many of the pieces that are bony in 

 the tortoise are replaced by simple cartilage or 

 membrane. 



The enormous weight of the shell of the turtle 

 would be a serious impediment to the motion of 

 this animal in the water, were there not some 

 provision made for diminishing the specific gra- 

 vity in the body. This purpose is answered by 

 the great capacity of the lungs, which, when 

 inflated with air, nearly fill the thorax, and 

 give great buoyancy to the whole mass. Thus, 

 wherever there exists a supposed inconvenience, 

 dependent on the fulfilment of one condition, 

 we are certain to meet with a compensation 

 in the structure of some other part, and in 



