88 VARIATION IN SNAILS chap. 



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

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

 nemoralis var. majo7\ 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 neighbourhood 

 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. alpestris^ 

 which is scarcely two-thirds the size of the type, occurs 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 jyisana 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 Gil tar 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. alpes- 

 tris^ of H. arhustorum mentioned above, besides being character- 

 istic of high Alpine localities, also occurs abundantly in low 

 marshes at Hoddesdon on the River Lea. Helix p)ulchella var. 

 costata, according to Jeffreys, is found in dry and sandy places, 

 often under loose stones and bricks on walls, while other author- 

 ities 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 Lim.iaea peregra inhabiting a certain pond near 

 Geneva, were, during one season, afflicted with a malform.ation 

 of the base of the columella. Tliis deformity coincided with 

 the appearance, in the same waters, of extraordinary numbers of 

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



1 J. G. Jeffreys, British Conchology, vol. i. p. 214. 



