THE ADDER. 



97 



marking, the two dark limbs of the V dividing to 

 enclose it. This plate is very often a well-marked 

 hexagon. But even this large plate is very variable, 

 and sometimes not much bigger than the others. 



The scales which cover the back and sides are small, 

 and all keeled or carinated. Those on the belly are 

 broad, and of very varied colours, 

 according to the sex. They are 

 disposed in a single row until 

 the tail is reached, when the 

 row becomes a double one, the 

 two tail scale rows overlapping 

 each other in the middle line. 

 The last inch or so of these 

 caudal scales is frequently of 

 a bright orange colour on the 

 ventral surface. 



Jaws and fangs. — The whole 

 mechanism of the adder jaw has 

 been developed in the direction 



of perfecting the poison apparatus, and a very wonder- 

 ful mechanism it is. Apart from this there is the same 

 mobility which we saw in the jaws of the ring snake 

 to allow of the swallowing of large articles of diet. 

 This is attained by the quadrate bones, which connect 

 the lower jaw with the skull, being movable ; and also 

 by the free movement of the jawbones with the palate- 

 bones. Tlie lower jaw is furnished with the usual 

 recurved teeth, while the upper jaw, a very short 



G 



Fig. 23. — Head Plates 

 OF Adder. 



(S, supra-ocular ; F, frontal ; 

 P, parietal ; C, cautlials.) 



