VARIATION IN SNAILS 



small depressed form, occurs at ' Sandwich and Falmouth.' ^ 

 Sometimes, however, the exact opposite is the case, for H. 

 nemoralis var. major, which is ' much larger ' than the type, 

 occurs on ' sandhills and downs ' and is ' remarkably large in the 

 I. of Arran, Co. Galway.' The dwarf form of Limnaea peregra 

 known as maritima appears to be confined to the neighbom^hood 

 of the sea. 



Dwarfing of the shell seems frequently to be the result of an 

 elevated locality, not perhaps so much as the direct consequence 

 of purer air and less barometric pressure, as of changes in the 

 character of the food supply and in the humidity of the air. 

 Several species of Helix have a variety minor which is charac- 

 teristic of an Alpine habitat. Helix arhustorum, var. aljpestris, 

 which is scarcely two-thirds the size of the type, occm^s on the 

 Swiss Alps in the region of perpetual snow. Sometimes a very 

 slight elevation is sufficient to produce the dwarfed form. At 

 Tenby the type form of Helix pisana is scattered in countless 

 numbers over the sandhills just above high-water mark. At the 

 extreme western end of these sandhills rises abruptly to a 

 height of over 100 feet the promontory known as Giltar Head, 

 the vegetation of which is entirely distinct from that of the 

 burrows below. There is a colony of H. pisana at the end of 

 Giltar, all of which are devoid of the characteristic markings of 

 the typical form, and most are dwarfed and stunted in growth. 



Occasionally the same variety will be found to be produced 

 by surroundings of very different nature. Thus the var. alpestris 

 of H arhistorum mentioned above, besides being characteristic 

 of high Alpine localities, also occurs abundantly in low marshes 

 at Hoddesdon on the River Lea. Helix p)ulc]iclla var. costata, ac- 

 cording to Jeffreys, is found in dry and sandy places, often under 

 loose stones and bricks on walls, while other authorities have 

 noticed it in wet and dry localities quite indifferently. 



Sometimes the production of a variety may be traced to the 

 intrusion of some other organism. According to Brot, nine- 

 tenths of the Limnaea peregra inhabiting a certain pond near 

 Geneva, were, during one season, afflicted with a malformation 

 of the base of the columella. This deformity coincided with the 

 appearance, in the same waters, of extraordinary numbers of 

 Hydra viridis. The next season, when the Hydra disappeared, 



^ J. G. Jeffreys, British Conclwlogy, vol. i. p. 214. 



