II ENDURANCE OF HEAT AND COLD 25 



locality at 15,000 ft., while Lminaea Hookeri has been taken 

 from over 16,400 ft. in Landour. In the Andes of Pern and 

 Bolivia, five species of Bulimulus, one of Pupa, and one of Limax 

 occur at an elevation of 10,500 to 15,000 ft. Several fresh- 

 water Mollusca inhabit Lake Titicaca, which stands at a height 

 of 12,550 ft. in the Bolivian table-land. 



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 

 desertoTum, 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 fiuviatilis lives at Bagneres de Bigorre 

 in water at about 68° F. In another hot spring in the eastern 

 Pyrenees a Bithynia lives at a temperatm^e 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 hiberna- 

 tion, 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 



