In certain parts of the desert of Algeria, where there is not a trace of vegetation to be seen, and the temperature at mid-day is 110° F., the ground is sometimes so covered with _Helix lactea_ as to appear perfectly white. Dr. F. H. H. Guillemard has told me that he noticed, in somewhat similar surroundings between Fez and Tangier, _H. pisana_ in such extraordinary abundance that they hung from the low scrub in bunches the size of a man’s two fists. It is singular that Mollusca should live, and not only live, but flourish, in localities apparently so unpromising. Shells which occur in the Algerian Sahara are actually larger and altogether finer than the ordinary European form of the same species. In order to protect themselves to some extent against the scorching heat and consequent evaporation, desert species are frequently modified in one of two ways; the shell becomes either white or a light dusky brown, as in the familiar _Helix desertorum_, or else it gains immensely in thickness. Specimens of _H. pomatia_, recently procured from Fez, are of extraordinary thickness as compared with forms from our own chalk downs of Kent and Surrey. Fresh-water Mollusca are frequently found inhabiting hot springs. Thus _Neritina fluviatilis_ lives at Bagnères de Bigorre in water at about 68° F. In another hot spring in the eastern Pyrenees a _Bithynia_ lives at a temperature of over 73° F.; while Blainville mentions another case of a _Bithynia_ living in water at 122° F. =Hibernation and Aestivation.=--As autumn begins to draw on, and the first frosts to nip vegetation, terrestrial species retire beneath stones, into cracks in old walls, holes in tree trunks, deep fissures in rocks, and nooks and crannies of every kind, or else bury themselves deeply in the earth or in moss and heaps of leaves. They thus commence their period of _hibernation_, which varies in length according to the duration of winter. Frequently masses of Helices may be found attached to one another, probably not so much for the sake of warmth, for their temperature is but low, as to share the comforts of a cosy retreat in common. Slugs generally hibernate alone, excavating a sort of nest in the earth, in which they encyst themselves, contracting their bodies until they are almost round, and secreting a covering of their own slime. The Helices usually close up the mouth of their shell by the formation of a membranous or chalky _epiphragm_, which will be further described below. Both snails and slugs take care to be in good condition at the time their winter sleep begins, and for this reason the former are said to be most esteemed by foreign epicures if captured just at this period.[38]