Chap, i x.i SKULL OF OPHIDIA. 339 



the maxilla by the transverse bone (tr). The long 

 quadrate is movable in a plane parallel to the Icng 

 axis of the skull, and when the mouth is shut is 

 directed backwards, so that the palato-pterygoid bar 

 forms a straight line, while the palatine and transverse 

 bones pull the maxilla backwards and upwards, and 

 so cause the poison fang to lie along the floor of the 

 skull. When the mouth opens, the point of attach- 

 ment of the lower jaw is necessarily drawn upwards ; 

 this action forces the lower end of the quadrate 

 forwards ; the quadrate acts on the pterygoid, and the 

 pterygoid on the palatine and transverse bones, which, 

 driving forwards what is before them, force the 

 maxilla outwards and downwards ; the poison fang is 

 thus caused to become vertical, or in a position to 

 inject the poison, which is simultaneously forced out 

 of its gland through the channel in the tooth, and so 

 into the body of the victim. 



In the poisonous Mexican lizard Heloderma, the 

 poison glands are modifications of sublingual glands, 

 and the lower jaw is traversed by the channels 

 through which their ducts pass into the floor of the 

 mouth. In Hatteria, the teeth are lost after a 

 certain time, and their place is taken by the bones 

 that bear them, the margins of which are sharp and 

 of exceedingly dense structure, so that here the bones 

 take the place of the teeth. (See page 146.) 



While in the Ophidia the two halves of the lower 

 jaw are connected by ligament, and in most Lizards 

 and all Crocodiles by sutures, they become completely 

 fused with one another in the Chelonia ; the same 

 phenomenon is to be observed in Birds, and in both 

 cases it is, no doubt, to be correlated with the loss of 

 teeth and the acquirement of a horny investment or 

 foeak. In the turtles the fossa on either side of the 

 skull, bounded externally by the jugal and quadrato- 

 jugal, is roofed over by an outgrowth of the parietal ; 



