VARIATION IN SNAILS 89 



Uie next generation of Limnaea was found to have resumed its 

 normal form. 



It has been noticed that a form of Helix caperata with a 

 flattened spire and wide umbilicus is restricted to tilled fields, 

 especially the borders of clover fields, while a form with a more 

 elevated spire and more compact whorls occurs exclusively in 

 open downs and uncultivated places. The Eev. S. S. Pearce 

 accounts ^ for this divergence by the explanation that the flatter 

 spire enables the shell of the fields to creep about more easily 

 under the leaves or matted weeds, seldom requiring to crawl up 

 a stalk or stem, while on the short turf of the downs and 

 pastures the smaller and more rounded shell enables the animal 

 to manoeuvre in and out of the blades of grass, and even to 

 crawl up them with considerable activity. The same writer 

 endeavours to explain the causes which regulate the distribution 

 of H. caperata var. ornatci. He found that tliis variety (dark 

 bands on a white ground) occiu-red almost exclusively on downs 

 which were fed upon by sheep, associated with the common or 

 mottled form, while the latter form alone occurred in localities 

 where sheep were not accustomed to feed. Assuming then, as 

 is probably the case, that sheep, in the course of their close 

 pasturing, devour many small snails, he believes that individuals 

 of the more conspicuous form ornata were more likely to be 

 noticed, and therefore avoided, by the sheep, than the mottled 

 form, which would more easily escape their observation. Hence 

 the var. ornata is due to the advantage which strikingly coloured 

 individuals obtained owing to their conspicuous habit, as com- 

 pared with the typical form, which would be less readily 

 detected. 



(b) Changes in Soil, Station, Character of Water, etc. — A de- 

 ficiency of lime in the composition of the soil of any particular 

 locality produces very marked effects upon the shells of the 

 Mollusca which inhabit it ; they become small and very thin, 

 occasionally almost transparent. The well-known var. tenuis of 

 Helix asioersa occurs on downs in the Channel Islands where 

 calcareous material is scarce. For similar reasons, H arltistorum 

 develops a war.fusca, which is depressed, very thin, and trans- 

 parent, at Scilly, and also at Lunna I., E. Zetland. 



The common dog-whelk {Purpura lapillus) of our own coasts 



^ Journ. of Conch, vi. p. 1 23. 



