296 TAYLOR : ON thp: variation of mollusca. 



occurs without a reason and no effect without a cause, we 

 must in those cases store away facts and observations to 

 which we may hope sooner or later to find the key, as I appre- 

 hend it is an important part of our study to endeavour to dis- 

 cover and make plain the laws which govern variation. 



I will first allude to some of the effects of geological con- 

 formation, thus : — Hermann Jordan in speaking of the influence 

 of limestone says, that in the Southern districts of Europe, where 

 favorable climatic conditions to mollusca are more universal 

 than in the Northern and more inclement countries, mollusks 

 are not so much confined or restricted to limestone soils as in 

 more Northern regions, and he ascribes their greater partiality 

 to calcareous ground, not so much to its chemical composition 

 as its greater power of absorbing warmth, and because it gives 

 usually a greater variety ofphysical conditions than do the primi- 

 tive rocks. Clessin points out thkt the results of a deficiency 

 of limestone are not only visible in the thinness and size of the 

 shell, but that even the form of some species is affected, thus 

 he states that the Clausili^ have the shell shorter than usual, 

 and that Helix lapicida becomes somewhat rounded on the 

 periphery. However these may be, it is certain that the deficiency 

 of limestone leads to the production of very thin, fragile and 

 horny shells, which under more favourable conditions are stoutly 

 built and strongly coloured. A well known and striking instance 

 is the remarkably delicate form of the common ^Q}Ci H. aspersa, 

 which is plentiful in some parts of Guernsey. 



Peat moors, on account of the absence of calcareous 

 matter, have been remarked to produce dwarfed, thin and fragile 

 shells and often these are sufficiently diverse to form very peculiar 

 and marked varieties. 



Among freshwater shells some species like Unio margariti- 

 ferus and Neritina seem to have a superior power of withdraw' 

 ing from the water to form their shell, the calcic carbonate it 

 may contain, and thus in the granitic region where this sub- 

 stance is very limited, they produce solid and thick shells, while 



J.C, v., April, 1888. 



