CHAPTEK I. 



Number and Distribution of British Mollusca Classification 

 Bivalves and Univalves Aquatic and Terrestrial Univalves- 

 Commoner forms of Shells. 



THE number of British land and freshwater 

 mollusca is somewhere about one hundred and 

 twenty, excluding such as have been evidently 

 introduced, or described as British on doubtful 

 authority, but including the slugs, which, though 

 generally regarded as shell-less, have the shell 

 either rudimentary, and of an indefinite form, or 

 shield shape, placed beneath the mantle. 



Of these one hundred and twenty, about seventy- 

 five are terrestrial, and forty-five aquatic; but it 

 is, of course, to be understood that, in speaking 

 of them as British, we do not mean to imply that 

 they are not found out of the British Islands. 

 So far from this being the case, the majority of 

 them are commonly distributed over the north of 

 Europe, while many are found not only in Southern 

 Europe, North Africa, and Asia Minor, but even 



