1400 THE STORY OF THE UNIVERSE 



broad plates immovably united to the spine, when 

 they have proceeded a certain distance separate from 

 each other and resume their usual form; the inter- 

 vening spaces between two adjacent 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 intervals between its imperfectly developed 

 elements also membraneous. 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 aquat- 

 ic Chelonia and those that live on land are still more 

 strongly marked in the genus Trionyx, or soft tor- 

 toise, 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 gravity 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 incon- 

 venience, dependent on the fulfilment of one condi- 

 tion, we are certain to meet with a compensation in 

 the structure of some other part and in the mode of 

 executing some other function. An express pro- 

 vision for giving buoyancy has been made in the con- 

 struction of the shell of a species of tortoise inhabit- 



