DIVISIOXS OF KEPTILIA. 261 



CHAPTER 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 tlie jaws ai-e not jjrovided with teeth, but are encased 

 in horn, so as to form a kind of beak. The case in which the body 

 of a C!helonian is protected is composed partly of integumentaiy 

 plates and partly of flattened bones belonging to the true skeleton, 

 and it is composed essentially of two jiieces, one placed (jn the back 

 and the other on the lower surface of the l>oily, 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 comjjletely flat or concave, and is called the plaxtrort. 

 The carapace and plastron, as just said, are united liy 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 S|iinous 

 processes of the vertebne, and the greatly dereloped 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 ]>lastron is also composed partly of bony and ])artly 

 of horny plates, but opinions ditter 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 peculiai'ities with regard to the skeleton which 

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

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

 cjuite inflexible ; secondly, that the heads of the rilis are articulated 

 directly to the bodies of the vertebiw ; and, thinlly, that the 

 scapular and pelvic arches, supporting resijectively the fore and 

 hind limbs, are situated irithiii the carajiace (Hg. 187), so that the 

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