28 THE STORY OF REPTILE LIFE. 



In the structure of the skull several points 

 are to be observed whereby the Chelonia differ 

 from other reptiles. With these, in detail, we 

 have no concern here ; for our purpose it is 

 enough to notice, firstly, that teeth are con- 

 spicuous by their absence. Their work is per- 

 formed by horny sheaths which encase the jaws 

 as in birds. That the ancestral Chelonia had 

 teeth is very probable, and doubtless some day 

 this fact will be established by the discovery of 

 a fossil skull with teeth implanted in the jaws. 

 In one other point the Chelonia and the birds 

 agree though this of course by no means im- 

 plies relationship and this is in the form of the 

 lower jaw, which, instead of being made up of 

 two separate halves, is fused into a single bone. 



The breathing of the Chelonia has acquired 

 certain peculiarities, inasmuch as, on account of 

 the rigid walls of the shell, expansion of the 

 chest cavity by the movement of the ribs and 

 abdomen has become impossible. The lungs, 

 which are complicated, spongy structures, are 

 filled and emptied partly by the movement of 

 the neck and limbs, which by their movement 

 act as pistons, and partly by the action of the 

 tongue bones, which are of great size. By these, 

 when the neck is stretched out, the throat is 

 alternately inflated and emptied by air drawn in 

 through the nostrils. The deflation of the throat 

 causes the air to be forced down the windpipe, 

 the valves of the nostrils preventing its escape 

 by any other way. 



In every great group of animals we find that 

 the struggle for existence has caused a gradual 



