The immediate neighbourhood of the sea appears frequently to have the effect of dwarfing land Mollusca. Thus the var. _conoidea_ of _Helix aspersa_, which is small, conical, with a compressed mouth, occurs ‘on sandhills and cliffs at the seaside.’ The varieties _conica_ and _nana_ of _Helix hispida_ are found ‘near the sea.’ _Helix virgata_ is exceedingly small in similar localities, and tends to become unicoloured. _H. caperata_ var. _Gigaxii_, a small depressed form, occurs at ‘Sandwich and Falmouth.’[193] 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 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 characteristic of an Alpine habitat. _Helix arbustorum_ 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 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. arbustorum_ mentioned above, besides being characteristic of high Alpine localities, also occurs abundantly in low marshes at Hoddesdon on the River Lea. _Helix pulchella_ var. _costata_, according 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, the next generation of _Limnaea_ was found to have resumed its normal form.