CHELONIAN REPTILES. 421 



carapace is smaller, has a more flattened form, 

 and cannot afford protection to the head and limbs. 

 These latter organs are proportionally larger, pre- 

 sent a greater developement of the radins and ulna, 

 and are compressed into a flat expanded surface. 

 Previously to the retraction of the head and limbs 

 M'ithin the shell, the air is expelled from the large 

 cavities of the lungs, by the vigorous actions of the 

 abdominal muscles, which exist in these animals 

 as well as in all the vertebrata, although here they 

 are covered by the bones, and compress the lungs 

 by pushing the abdominal viscera against them. 

 This sudden expulsion of air is the cause of the 

 long continued hissing sound, which the tortoise 

 emits while preparing to retreat into its strong 

 hold. 



The ribs, though they at first assume the form 

 of broad plates, immoveably united to the spine, 

 when they have proceeded a certain distance, se- 

 parate from each other, and resume their usual 

 form ; the intervening spaces between two adjacent 

 ribs being here filled up by membrane. The plas- 

 tron 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 membranous. 

 All this renders the whole shell less compact, more 

 flexible, and weaker; but the movements of tlie 

 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 Trioiiyx, 

 or soft tortoise; which is destitute of scales, and in 

 which many of the pieces that are bony in the 



