THE OPHIDIA IN THE M ONNOW VALLEY. 205 



this theory ; and the fact that the adder is so often 

 restricted to a particular locality in many counties, in 

 addition to what we have seen to account for its 

 presence here, leads rather to the conclusion that it is 

 a case simply of a suitable environment. 



The result of this investigation into the distribution 

 of the Ophidia in the valley of the Monnow in its 

 course on the South Herefordshire boundary may be 

 summed up thus : — 



1. That the adder (Vipera berus) is here relatively 



common. 



2. That the average length is 24 inches in the adult 



male, 25h in the adult female — being much 

 above the average of most localities. 



3. That the ring snake {Trojndonotus natrix) is not 



found, except probably when artificially con- 

 veyed thither, and certainly does not breed there. 



4. That the smooth snake does not occur. 



5. The small red viper has been taken once (by the 



author), and seen on two other occasions.^ 

 Some such factors as the foregoing must be held to 

 be mainly responsible for the striking ophidian char- 

 acteristics of this and other localities which present 

 similar peculiarities. There may, of course, be other 

 modifying influences at work, which have either in 

 past times contributed to, or even now are maintain- 

 ing, the existing state of affairs. 



1 Cf. "The Reptilia of the Monnow Valley," Woolhope Club 

 TrauBRctions, 1898-99.— Author. 



