CHAPTER xxvnr. 



DIVISIONS OF EEPTILIA. 



Oebee I. Chelonia (Gr. chelone, a tortoise). — In this 

 order are included tlie various Tortoises and Turtles, charac- 

 terized by having the body enclosed in a bony case or box, 

 and by the fact that the jaws are not provided with teeth, but 

 are encased in horn, so as to form a kind of beak. The case 

 in which the body of a Cheloniau is protected is composed 

 partly of integumentary plates, and partly of flattened bones 

 belonging to the true skeleton, and it is composed essentially 

 of two pieces, one placed on the back and the other on the 

 lower surface of the body, firmly united together at their edges. 

 The dorsal shield is more or less convex and rounded, and is 

 called the carapace (Fig. 118, ca) ; while the ventral shield is 

 more or less completely flat or concave, and is called the plas- 

 tron. The carapace and plastron, as just said, are united by 

 their edges, but they leave two openings, one in front for the 

 head, and one behind for the taU. The carapace is essentially 

 composed of the flattened and expanded spinous processes of 

 the vertebrse and the greatly-developed ribs, covered by a 

 series of horny plates. These are growths of the integument, 

 and in some cases they constitute the " tortoise-shell " of com- 

 merce. The plastron is also composed partly of bony and 

 partly of horny plates, but opinions differ as to whether the 

 bony plates are to be looked upon as formed by an expanded 

 breastbone, or whether they are merely integumentary, the 

 probabilities being in favor of the latter view. 



The remaining peculiarities with regard to the skeleton 

 which deserve special mention are : Firstly, that the dorsal 

 vertebras are immovably connected together, so that this region 

 of the spine is quite inflexible ; secondly, that the heads of the 

 ribs are articulated directly to the bodies of the vertebrES • and 



