CHELONIAN REPTILES. 329 



the intervals between its imperfectly developed elements 

 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 Ihe aquatic Che- 

 Ionia and those that live on land are still more strongly- 

 marked in the genus Trionyx, 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 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 

 inconvenience, 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 o^her function. An express provision for giving buoy- 

 ancy has been made in the construction of the shell of a spe- 

 cies of tortoise inhabiting the coasts of the Scychelle Islands. 

 The under surface of the shell, instead of being gently con- 

 cave, as in land tortoises, has a deep circular concavity in 

 the centre, above four inches in depth, which, when the 

 animal goes into the water, retains a large volume of air, 

 buoying up the whole mass while it remains in that ele- 

 ment* The greater size of turtles, when compared with 

 tortoises, is a farther instance of the superior facility with 

 which organic growth proceeds in aquatic than in land ani- 

 mals formed on the same model of construction. 



* Home's Lectures, vi. 37. 



Vol. I. 42 



