REPTILES 1897 



the situation of the ribs is the consequence of the 

 change in their office. When designed to be very 

 movable, we find them attached either to the ex- 

 tremities of the transverse processes or to the articu- 

 lar surfaces of a single vertebra; but where solidity 

 and security are aimed at, they are always inserted 

 between the bodies of two vertebrae. It is remark- 

 able, indeed, that a great number of the peculiarities 

 which distinguish the conformation of the chelonia 

 from that of other reptiles indicate an approach to 

 the structure of birds; as if Nature had intended this 

 mall group of animals to be an intermediate link 

 of gradation to that new and important type of ani- 

 mals destined for a very different mode of existence. 

 It is to be noticed, also, that as the plates, which 

 form this investing case, are bony structures, they 

 could not with any safety have been exposed to the 

 action cxf the atmosphere^ Hence we find them 

 covered throughout with a thin, horny plate, origi- 

 nally a production of the integument. This substance 

 is commonly known by the name of tortoise shell. 



The immobility of the trunk is compensated, as 

 far as regards the safety of the head, by the great 

 flexibility of the neck; which is composed of seven 

 vertebrae, unencumbered by processes, and capable 

 of taking a double curvature like the letter S, when 

 the head is to be retracted within the carapace. These 

 vertebrae are joined by the ball and socket articula- 

 tion common to all the existing species of reptiles.* 



* The expression of this fact is thus qualified, because it 

 does not apply to many fossil or extinct species, such as the 

 Ichthyosaurus. 



