BOYCOTT : SIZE VAEIATION OF CLAUSILIA AND ENA. 35 



for some moss in a few places and a scattered growth of Cephalanthera 

 generally. The tree-trunks are also, with rare exceptions, free from 

 moss or any but a scanty growth of lichens. The wood runs roughly 

 east and west (see sketch-map) ; its southern edge nearly corresponds 

 with the highest part of the ridge, while its northern part lies on a 

 steep slope leading to a narrow valley, with a second wood beyond. 

 The prevailing winds being from the south-west, the upper parts of 

 the wood are exposed, while the northern slope and the valley 

 beneath are much more sheltered. In a general way the whole 

 wood would usually be considered a single homogeneous locus, and 

 specimens collected in one part would not be separated from those 

 from another part. 



§ 3. The present inquiry was made to test this presumption by 

 finding out whether Clausilia bidentata from one part of the wood 

 was larger or smaller than from another part ; incidentally, Ena 

 obscura was also examined less fully. 



To this end collections were made in five different areas (see map, 

 p. 34), as follows : — ■ 



A : Six trees in a line 26 yards long in the valley, and very 

 sheltered. 



B : Thirteen trees in a rough circle of about 23 yards, 200 yards 

 south-west of A, and some 120 feet higher, nearly on the top of 

 the hill. 



C : Twenty-one trees in 27 by 15 yards, 50 yards north and 

 west of B, a little lower and more sheltered behind the hill-top ; 

 some moss on ground and trees. 



D : Twenty- two trees in 41 by 30 yards, 120 yards west of C ; 

 lying on a steep slope, the difference in level between top and bottom 

 being about 40 feet. 



E : Twenty trees in a triangle of about 25 yards, 320 yards west 

 of D ; low and sheltered. 



The shading varied to some extent ; A was the lightest area, 

 with thin trees to the south and none to the north. C and D are both 

 open to some extent owing to the steep slope to the north. At B 

 the trees are rather thin to the south, close on the other sides. E is 

 the darkest locus. In three of these areas collections were made 

 from individual trees as well as from the area as a whole, i.e. from 

 six trees in area A, from three in area B, and from six in area D : 

 their relative positions are shown in diagram 1. On an average 

 there is one tree to about 35 or 40 square yards of ground. 



The snails were collected as opportunity offered on various 

 occasions between June 19 and December 2, 1918, as they crawled 

 or sat upon the trunks, all the specimens found on the selected trees 

 being taken without selection. The measurements and computations 

 were made as already described.^ A certain number of shells had to 

 be excluded from measurement on account of decollation, in all 

 ^ Journal of Conch., vol. xvi, 1919, p. 11. 



