INTRODUCTION. 23 



in the rest of Europe, and, secondly, the extent to 

 which the various species are diffused over the differ- 

 ent parts of the island, which is influenced by the cli- 

 mate, 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. Helix fasca Montagu. 



3. Vertigo angustior. 



4. Amphipeplea involuta. 



5. Planorbis glaber. 



6. Pisidium cinereum. 



7. nitidum. 



8. pulchellum. 



9. Henslowianum. 



Most of these are newly-described species, and may 

 have been overlooked, or only considered as varieties 

 of other well-known species, by the zoologists of the 

 Continent. It is to be remarked 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 crys- 

 tallinay 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. 



