DIVISIONS OF EEPTILIA. 261 



CHAPTEE XXVII. 



DIVISIONS OF EEPTILIA.. 



Order I. Chelonia. 



In this order are included the various Tortoises and Turtles, char- 

 acterised 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 Chelonian 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; whilst the ventral shield 

 is more or less completely flat or concave, and is called the pldstron. 

 The carapace and plastron, as just said, are united by their edges, 

 but they leave two openings, one in front for the head and fore- 

 limbs, and one behind for the tail and hind-limbs. 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 (fig. 188) they constitute the "tortoise-shell" of 

 commerce. 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 favour of the latter view. 



The remaining peculiarities with regard to the skeleton which 

 deserve special mention are : Firstly, that the dorsal vertebrse are 

 immovably connected together, so that this region of the spine is 

 quite infiexible ; secondly, that the heads of the ribs are articulated 

 directly to the bodies of the vertebrae ; and, thirdly, that the 

 scapular and pelvic arches, supporting respectively the fore and 

 hind limbs, are situated within the carapace (fig. 187), so that the 



