REPTILIA, 443 
are loosely united by ligament. Hence the snakes have an 
enormous “‘ gape,” and can ‘‘swallow” entire animals which 
exceed their own diameter. All Op/idia are carnivorous. 
The non-poisonous groups usually have two rows of Jong 
recurved teeth on the maxillz and the palatines and ptery- 
goids respectively. Between these rows fits the row of teeth 
on the mandible. In the poisonous group the maxilla is 
freely hinged, and bears a single large fang or grooved tooth 
connected with the poison-gland, a modified salivary gland. 
There are also a few teeth on the pterygoids, palatines and 
mandibles. On closing its jaw, the snake’s maxilla with its 
Fig. 313-—RIGHT SHOULDER GIRDLE OF A TORTOISE. 
fi.) Scapula. 
; <\ Precoracoid. 
f Glenoid. 
fang is swung” up into the roof of the mouth by the automatic 
movement of the quadrate, pterygoid, transverse bone and 
maxilla. 
The snakes may therefore be said to exhibit extreme 
specialisation for a unique method of locomotion, involving 
loss of limbs and limb-girdles, and for an equally remarkable 
method of feeding. 
ORDER LV.—Chelonia. 
The Chelonia comprise the tortoises and turtles. They 
have an exoskeleton of horny epidermic plates, to which is 
added an underlying dermal layer of bony scutes. The whole 
