24 



BRITISH SERPENTS. 



plates on either side of these large ones. Notice that 

 the eyes are opposite the row of three plates, and that 

 the largest of all are the two immediately behind 

 these. This is different from 

 the arrangement in the adder, 

 but cannot be said to be a very 

 ready means of distinguishing 

 the species until it has been 

 captured. The scales on the 

 back are keeled or carinated — 

 that is, they ar^ possessed of a 

 mid-rib like a leaf. Hence the 

 generic term of " Tropidonotus," 

 which means literally " keel- 

 backed." This keel is still more 

 prominent in the adder. The 

 scales along the ventral surface 

 are in a single row as far as 

 the anal orifice, when the row 

 becomes a double one. The tail 

 is about one-fourth the length of 

 the whole reptile. 

 The teeth are in two rows on both the upper and 

 lower jaws, and are recurved, pointing to the throat, so 

 that it is difficult to extract anything from the mouth 

 when the jaws have closed upon it. " They are not in 

 sockets, and consequently are not used to tear or 

 crush food." ^ 



^ Packard. 



Fig. 5. — Head Plates 

 OF Ring Snake. 



(N, nasal ; F, frontal ; S, 

 supra-ocular; P, parietal.) 



