TAYLOR : ON THE VARIATION OF MOLLUSCA. 299 



it extends over a period of more than 20 years — to investigate 

 the subject. The result of his labours and experience clearly 

 demonstrate that instead of spending the summer on the tops 

 of the trees, it is in dry weather buried in the ground and 

 among loose stones, in some instances in the latter situation, 

 being even 2 feet below the surface. Like its ally Bulinius 

 obscurus and other species, it is susceptible of the effects of 

 moisture, and I have myself seen it, in company with other 

 species, mounting the beech trees to a considerable height 

 immediately after heavy rain. 



The effects of temperature have been studied by Rodel, 

 who states that land and freshwater shells perish when exposed 

 for half-an-hour or so to about 17° of frost, or if the cold be 

 continued steadily for a couple of days 9° of frost is sufficient 

 to cause their death. Mature specimens resist the cold better 

 and withstand a degree or two more than young ones.. As we 

 should hardly be prepared to expect ^ thinly-clad or absolutely 

 naked mollusks withstand cold better than thickly shelled 

 species; Helix aspersa for instance possessing a thick and solid 

 shell is very sensitive to cold and retires early for the winter. 

 The most thinly clad of our land species, Vitrina pelhicida 

 is most active during the winter months, and has even been 

 noticed crawling briskly over the snow-covered herbage and 

 ground. 



The same law applies to the freshwater shells, the effects of 

 cold having been shown to be, that species which generally 

 present a somewhat strong shell under ordinary conditions, 

 secrete one of much greater thinness and fragility as well as of 

 a reduced size. Karl Semper has shown in Limnma stagnalis 

 that a temperature of 53° entirely stops the growth of the shell, 

 though the moUusk will continue to feed at a much lower tem- 

 perature, and we are led therefore to conclude that the energy 

 thus stored up is used in some other way than in the elaboration 

 of shell matter, possibly in resisting the depressing and destruc- 

 tive effects of too great a reduction of temperature. Limncea 



