A variety of _Limnaea peregra_ with a short spire and rather strong, stoutly built shell occurs in Lakes Windermere, Derwentwater, and Llyn-y-van-fach. It lives adhering to stones in places where there are very few weeds, its shape enabling it to withstand the surf of these large lakes, to which the ordinary form would probably succumb.[199] Scalariform specimens of _Planorbis_ are said to occur most commonly in waters which are choked by vegetation, and it has been shown that this form of shell is able to make its way through masses of dense weed much more readily than specimens of normal shape. Continental authorities have long considered _Limnaea peregra_ and _L. ovata_ as two distinct species. Hazay, however, has succeeded in rearing specimens of so-called _peregra_ from the ova of _ovata_, and so-called _ovata_ from the ova of _peregra_, simply by placing one species in running water, and the other in still water. According to Mr. J. S. Gibbons[200] certain species of _Littorina_, in tropical and sub-tropical regions, are confined to water more or less brackish, being incapable of living in pure salt water. “I have met,” says Mr. Gibbons, “with three of these species, and in each case they have been distinguished from the truly marine species by the extreme (comparative) thinness of their shells, and by their colouring being richer and more varied; they are also usually more elaborately marked. They are to be met with under three different conditions--(1) in harbours and bays where the water is salt with but a slight admixture of fresh water; (2) in mangrove swamps where salt and fresh water mix in pretty equal volume; (3) on dry land, but near a marsh or the dry bed of one. “_L. intermedia_ Reeve, a widely diffused E. African shell, attaches itself by a thin pellicle of dried mucus to grass growing by the margin of slightly brackish marshes near the coast, resembling in its mode of suspension the Old World _Cyclostoma_. I have found it in vast numbers in situations where, during the greater part of the year, it is exposed to the full glare of an almost vertical sun, its only source of moisture being a slight dew at night-time. The W. Indian _L. angulifera_ Lam., and a beautifully coloured E. African species (_? L. carinifera_), are found in mangrove swamps; they are, however, less independent of salt water than the last.”