REPTILES IN THE MAKING 179 



the head and neck, which, like the limbs and tail, can no 

 longer be drawn within the shelter of the bony fortress. 

 In some of the giant tortoises which live on islands where 

 there are no large carnivores, the shell has thus lost its 

 protective character and has grown exceedingly thin. 



Certain aquatic tortoises display interesting phases of 

 degeneracy in this connection. Thus in the " Mud-turtle," 

 or " Stink-pot Terrapin," the degeneracy has begun 

 apparently with the horny outer sheath of the breastplate. 

 This covering is composed, as in other tortoises, of several 

 symmetrically arranged plates, and in the newly-hatched 

 terrapin these have a normal arrangement. But as the 

 growth of the body proceeds, that of the horny plates 

 ceases. As a consequence they are gradually forced apart. 

 In the water tortoises or " soft-shelled turtles " of the 

 genus Trionyx, the upper part of the bony shell or back 

 shield has degenerated so far that it forms but little 

 more than a large island of bone in the midst of a great 

 expanse of leathery skin, the horny shields which form so 

 characteristic a feature on the back of the typical tortoise 

 having vaulted completely. 



The belly shield, curiously enough, seems to have de- 

 generated in like manner, and then to have undergone a 

 process of renewed vitality ; for in very young specimens 

 of such tortoises the several bony pieces which make up 

 the typical breastplate are represented by mere splints 

 embedded in skin. Later these bony bars thicken, broaden 

 out, and come almost to the surface, as if once more they 

 would assume the form of a bony, horn-ensheathed breast- 

 plate. 



