IxXXviii INTRODUCTION, [CH. 



The same basis of classification may be adopted for our 

 land and freshwater shells. These have been separated 

 by Forbes and Hanley into only three types, viz. North 

 European, Central European, and South European. The 

 third division of the foregoing category (viz. Oceanic) is 

 of course inapplicable to this group ; but in other re- 

 spects the principles which regulate their distribution 

 are nearly the same as in the case of their marine ana- 

 logues. The difference of aspect between these and 

 marine species, so far as regards their distribution, is very 

 noticeable, although, in this point of view, many of the 

 land and freshwater shells exhibit a greater resem- 

 blance to littoral species than to those which inhabit 

 deeper water, by reason of their external conditions. 

 Temperature or climate is one of the principal agents 

 in regulating the diffusion of land and freshwater Mol- 

 lusca ; and their limits are often sharply defined by a 

 strait of the sea or a mountain-range. Some conspi- 

 cuous land-shells (as Helix fruticum and H. incarnata) 

 live in the North of France, although they have never 

 been found in this country unless in a subfossil state 

 and as the relics of a past state of things. Some of our 

 common snails do not pass the Grampian Hills. In 

 Zetland the Helix aspersa is a total stranger, fortunately 

 for the poor gardens of the natives ; and only a scanty 

 remnant of the tribe have succeeded in crossing the 

 Pentland Firth or maintaining their existence in these 

 barren isles. Freshwater shells are not so restricted in 

 their distribution, although one of our native species 

 {Limncea involuta) has hitherto been discovered in only 

 one locality — assuming that this species is distinct from 

 L. glutinosa, which does not appear to have been found 

 in the same district. A table of distribution of the 

 land and freshwater shells which inhabit the British 



