1394 THE STCi.Y OF THE UNIVERSE 



quadrupeds, obliges them in general to rest the 

 weight of the trunk of the body on the ground, when 

 they are not actually moving. None of these reptiles 

 has any other kind of pace than that of walking 

 or jumping; being incapable of performing either a 

 trot or a gallop, in consequence of the obliquity of 

 the plane in which their limbs move. The chameleon 

 walks with great slowness and apparent difficulty; 

 and we have seen that, in consequence of the struc- 

 ture of the bones of its neck, the crocodile, though' 

 capable of swift motion in a straight line, is unable 

 to turn itself round quickly. The general type of 

 these reptiles, having reference to an amphibious 

 life, has not attained that exclusive adaptation to a 

 terrestrial existence which we find in the higher 

 orders of the Mammalia. 



The order of Chelonian Reptiles, which comprises 

 all the tribes of tortoises and turtles, appears to 

 constitute an exception to the general laws of con- 

 formation which prevail among vertebrated ani- 

 mals: for instead of presenting a skeleton wholly 

 internal, the trunk of the body is found to be in- 

 closed on every side in a bony case, which leaves 

 openings only for the head, the tail, and the fore 

 and hind extremities. That portion of this osseous 

 expansion which covers the back is termed the Cara- 

 pace; and the flat plate which defends the lower 

 part of the body is termed the Plastron. It is a form 

 of structure that reminds us of the defence provided 

 for animals very low in the scale of organization, 

 such as the echinus, the Crustacea, and the bivalve 

 mollusca. Yet the substance which forms these 



