INTRODUCTION. XXXlll 



claimed as truly confined to the British Islands. One re- 

 markable slug, the Geomalacus maculosus, has never been 

 met with out of the south-west of Ireland, and is there 

 exceedingly scarce. We think it not unlikely that this 

 curious animal will eventually be found to be a member of 

 the Lusitanian fauna. A very curious fresh-water shell, 

 Lymneus involutus^ is also, at present, confined to the same 

 portion of Ireland, and another species of the same genus, 

 L. Burnetii, is reputed peculiar to Britain. In these 

 instances we are possibly dealing with monstrosities, or 

 extreme varieties of the widely distributed L. pereger. One 

 fresh-water bivalve, a species of Pisidium, has not been 

 noticed beyond our limits ; but in this genus the characters 

 are too critical to warrant our inferring that it has not been 

 passed over abroad. Within our own areas, several of our 

 land and fresh-water shells do not range north to Scotland. 

 At least fifty species of our land and fresh- water Mollusks 

 may be regarded as generally distributed throughout Europe, 

 and as ranging both to the north and south of the British 

 Isles. None can be said to come from the north exclu- 

 sively. About eighteen are members of the fauna of North- 

 ern and Central Europe ; some fourteen are Central Euro- 

 pean species, not ranging far northwards or southwards ; 

 about sixteen belong to Central and Southern Europe ; not 

 more than four are decidedly of southern origin exclusively ; 

 some seven or eight species are reported to extend their 

 range into the New World ; but, except in the instances of 

 one or two aquatic types, they have probably been intro- 

 duced through the agency of man. Indeed, some of our 

 smaller snails have found their way through the same uncon- 

 scious assistance even into Australia. Several of our aquatic 

 species have a wide range into the further parts of the 

 Asiatic continent. The provinces of Asia and Africa that 



