42 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



§7. The connexion between the sizes and environments is more 

 obscure. It is suggestive that the largest specimens came from the 

 two most sheltered areas (A and E), while B, the most exposed, 

 yields the second smallest lot ; the more so because in the Portmadoc 

 series exposure was associated with small Clausilia hiclentata. In 

 North Wales, however, the densely shaded and sheltered loci also 

 yielded small specimens — a difference possibly due to the difference 

 in climate, close shelter on the chalk in Wilts giving agreeably damp 

 conditions, which are exaggerated on the less porous strata of 

 Portmadoc with a heavier rainfall to a degree of wetness which is 

 detrimental. " Shelter " and " exposure " may be presumed to 

 affect snails mostly by way of dampness ; the duration of moist 

 conditions after rainfall is greatly influenced by ventilation, and in 

 exposed places the time during which snails can move about is 

 considerably curtailed by rapid drying. 



§ 8. It is interesting to note that the local conditions which 

 influence decollation have no relation to those which influence size. 

 Note was taken of the number of decollated shells from each area ; 

 they are most ^ frequent in A, least in E. The natural presumption 

 is to look on decollation as an indication of vague unhealthiness, 

 but it is as likely associated with exuberant growth as with stunted 

 specimens. 



Total 2,994 39 1-3 



§ 9. Summary. — (1) Clausilia hidentata from small loci of similar 

 character within a few yards of one another do not usually differ 

 in size. 



(2) CI. hidentata and Ena ohscura from different areas in the same 

 wood 50 to 300 yards apart may definitely differ in size. 



(3) The size variation in the two species runs parallel. 



1 The differences A/C, A/D, and A/E alone are significant. 



