Chap. I. THE SHIELD. 255 



but Ijotli are still hmied, as it were, in the general mass of the ))()fly. On the 

 contrar}', in the ciiliuinating Reptile, the Turtle, the neck is completely free from 

 the mass of the body, and so also is the tail ; but there is still a sort of visceral 

 chest, inclosing the breast and abdomen. 



This general sketch of the essential characters of the Testudinata shows distinctly 

 that their most prominent features are also those which assign to them the highest 

 rank in their class. It is therefore plain, that the Testudinata, being a natural groujj, 

 constitute an order in the class of Reptiles, acknowledged to be such l)y most zoolo- 

 gists, while at the same time this typical group furni,shes additional evidence that the 

 characters I have considered above ^ as oi'dinal characters are marked out, as such, 

 in nature. It remains now to show, what is the degree of complication of their 

 structure which assigns to them that rank in their class. The comparison insti- 

 tuted here, between the leading groups of the true Reptiles and those of the 

 Batrachians, shows already the two series to consist equally of groups presenting a 

 natural gradation in their normal relations. We are, therefore, not only justified in 

 considering them all as natural orders, but this gradation, within their respective 

 limits, goes far also to show that the higher divisions under which they are com- 

 bined partake of the character of classes, and that Reptiles proper and Amphib- 

 ians are justly to be considered as two distinct classes. 



SECTION IV 



THE SHIELD. 



We have found the main ordinal character of the Turtles, in contradistinction 

 to other Reptiles, to consist in the nature of the dorsal column, which, in connec- 

 tion with other elements, forms in Turtles one continuous shield upon the back. 

 This dorsal shield, usually called by the French name "carapace," is connected 

 by a bridge with another shield, commonly called " plastron," which covers the 

 region of the breast and abdomen from l)elow. These two shields together form 

 a hard girdle around the soft organs of the trunk. 



If we take a Turtle of that family in which the idea or the type of Turtles is 

 carried out the furthest, namely, a land Turtle, we find these shields built up of two 

 very different elements, the skin and the true bony skeleton. If we analyze such 

 a .shield from the outside inwards, we see first a thick very hard and dry epidermis 



1 Part I., Clmi>. 2, Sect. 3, p. 150. 



