DIVISIONS OF REPTILIA 261 
CHAPTER XXVIII. 
DIVISIONS OF REPTILIA. 
ORDER I. CHELONIA.—In this order are included the vari- 
ous Tortoises and Turtles, characterised 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 con- 
vex and rounded, and is called the carapace (fig. 140, ca) ; whilst 
the ventral shield is more or less completely flat or concave, and 
is called the A/astron, 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 tail. The carapace is es- 
sentially composed of the flattened and expanded spinous pro- 
cesses of the vertebrae, and the greatly-developed ribs, covered 
by a series of horny plates. These are growths of the integu- 
ment, and in some cases 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 vertebree 
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 vertebree ; and, thirdly, 
that the scapular and pelvic arches, supporting respectively the 
