296 GEOGRAPHICAL DISTRIBUTION. 



12. Melania Matonii. Gray, Linn. Trans, vii. 150. t. 4. f. 6. 

 An African river shell. 



Thus have I felt myself called on to exclude from our Fauna 

 no fewer than 54 species. 



IV. 



GEOGRAPHICAL DISTRIBUTION. 



IN considering the geographical distribution of the British land 

 and fresh- water Mollusca, we must look at them at least 

 in two points of view : first, as regards their bearing on the 

 general distribution of Mollusca in the rest of Europe ; and 

 secondly, the extent to which the various species are diffused 

 over the different parts of the island, which is influenced by 

 the climate, the elevation, and the nature of the subsoil, or of 

 the rock of which the country is chiefly formed. 



All the species which appear to be really native, and are 

 therefore noticed in this work, are found in France, or in 

 different parts of Germany, except the following : 



1. Assiminia Gray ana. 2. Vertigo angustior. 3. Amphipe- 

 plea involuta. 4. Cyclas pattida. 5. C. pisidioides. 



These are newly described species, and may have been over- 

 looked, or only considered as varieties of other well-known 

 species, by the zoologists of the Continent. It is to be re- 

 marked that, in general, the British species, although they vary 

 among themselves, attain a moderate and nearly uniform size, 

 compared with those of the rest of Europe. Thus I have never 

 seen Lymneus stagnalis, Paludina achatina or crystalline^ so 

 large as those found in the south-eastern part of Germany ; or 

 Helix nemoralis, and other more common Helices, as those found 

 in Portugal, or the Helix aspersa from Algiers ; nor, on the 

 other hand, have I seen any English specimens of Helices so 

 stunted in growth as the Alpine varieties of H. nemoralis and 

 H. arbustorum, which I have received from the Swiss Alps. 



Ferussac has observed (Journ. de PJiys. 1820) that it is re- 



