34 



ON THE SIZE VARIATION OF CLAUSILIA BIDENTATA AND 

 EN A OBSGURA WITHIN A "LOCALITY". 



By Dr. A. E. Boycott, F.K.S. 



Eead dih January, 1920. 

 § 1 . It was shown in a previous communication ' that specimens 

 of Clausilia hidentnta from similar habitats in the same neighbourhood 

 could generally be readily distinguished from one another by 

 dijBerences in size. It was there shown that the snails living on one 

 stone wall were usually larger or smaller than those living on a similar 

 stone wall half a mile away, and therefore did not belong to precisely 

 the same familial group. A question which was not then examined 

 was how near together, in a habitat roughly homogeneous in 

 character, such distinguishable loci might be — a locus for any species 

 being an area throughout which that species is uniform in character. 

 Facilities, imperfect but tolerable, for collecting over an extended 

 period in a "Wiltshire beech-wood, gave an opportunity for making 

 some further inquiries into these questions. 



Tower Hill Plantation. 



§ 2. Tower Hill Plantation lies on a ridge of high chalk land, 

 two miles west of Boscombe, in south-east Wilts. It forms part of 

 a great ring plantation, and in its present form is presumably 

 modern, though the northern slope is too steep to have ever allowed 

 cultivation.^ In the parts with which we are concerned it is a 

 typical close-canopied beech-wood ; there is no ground flora except 



1 Journ. of Conch., vol. xvi, 1919, p. 10. 



^ I could find no signs of Ena montana, Limax cinereo-niger, or L. tenellus 

 which would have indicated an ancient wood ; even Helicigona lapicida was 

 absent, though it occurs a mile away in another wood. 



