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, 
aud 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. Fic. 23.—Herap PLates 
Jaws and fangs.—The whole oF ADDER. 
(S, supra-ocular 5; F, frontal 5 
mechanism of the adder jaw has eet 
. P, parietal; C, canthals.) 
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. The lower jaw is furnished with the usual 
recurved teeth, while the upper jaw, a very short 
G 
