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 Ophidia are carnivorous. 



The non-poisonous groups usually have two rows of long 

 recurved teeth on the maxillae 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 



T'ig- 313- — Right Shoulder Girdle of a Tortoise. 



\ Scapula. 



Glenoid. 



Coracoid. 



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 IV. — 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 



