SCHARFF : DISTRIBUTION OF BRITISH L. AND F. MOLLUSCA. 3 



neutralized by climate. As an example, he adduced the Shetland 

 Isles, where we have limestone, but nevertheless a great paucity of 

 species and individuals owing to the unfavourable climate. On the 

 other hand, he explained the presence of vast numbers of specimens 

 of Helix variabilis ( = virgata) and other species in Guernsey, where 

 the surface is composed of granite and quartz, by the overpowering 

 influence of climate over soil. The manner in which a particular 

 soil favours not only the distribution of the species, but also its 

 multiplication, has, I think, never been satisfactorily explained. To 

 demonstrate this action of the soil, a series of experiments would 

 have to be made by keeping a number of specimens (say, on a pure 

 quartz soil and on pure limestone) and allow them to breed under 

 similar conditions, recording the result from year to year. No doubt 

 we read a great deal in conchological works about certain " lime- 

 stone species," that is to say, species supposed to be confined to 

 limestone soil ; but I fail to see that there is any real foundation for 

 this belief. Some species, indeed, seem to increase at an enormous 

 rate on limestone, whilst only very few specimens can be found on a 

 slate or granite rock ; but it seems very probable that any mollusc can 

 thrive perfectly on the merest traces of lime, which it may find in 

 any kind of soil. 



It seems to me that the climate and soil in almost every part 

 of the British Islands is eminently suited for the development of 

 molluscan life, and the extreme poverty of our fauna must be 

 explained by other reasons than those put forward by Prof. Forbes. 

 At the time when he published his account of the pulmoniferous 

 mollusca, 101 species of non-marine mollusca were known to inhabit 

 the British Isles. Nearly forty more have been added since, and 

 many more may yet be discovered in some of the less known 

 regions. 



Prof. Forbes divided the British Isles into ten districts, according 

 to the distribution of their mollusca fauna, as follows : — I., the 

 Channel Isles; II., South-east of England (inclusive of Cambridge- 

 shire) ; III., South-west of England ; IV., North-east of England ; 

 V., North-west of England (inclusive of Isle of Man) ; VI., North 

 of Ireland ; VII., South of Ireland; VIII., South of Scotland; 

 IX., North of Scotland; X., Shetland Isles. 



The first district is founded on the occurrence of Helix naticoides 

 ( = H. aperta) and Helix revelata, but as the first of these has never 

 been found since, and may, according to Mr. Jeffreys' suggestion, 

 have been only the var. tennis of H. aspersa, and as the second 

 occurs in the south-west of England also, the distinct nature of the 

 district cannot be maintained. 



