163 



NOTES ON THE DISTRIBUTION OF BRITISH LAND AND FRESH- 

 WATER MOLLUSCA FROM THE POINT OF VIEW OF HABITAT 

 AND CLIMATE. 



By Dr. A. E. Boycott, F.E.S. 



Bead Mth January, 1921. 



PLATES V AND VI. 



The facts dealt with here are those accumulated by the late 

 W. Denison Roebuck for his " Census of distribution ". The plan 

 of the work and the details of the results obtained will be found in 

 the "Journal of Conchology ", vol. xvi, p. 165 ; it is enough here to 

 note that the records are in all cases based upon the examination 

 of actual specimens by the referees of the Conchological Society. 



The distribution of our British snails has been dealt with pretty 

 fully as a historico-geographical problem involving their past 

 history, their routes of migration, and so forth. My present purpose 

 is to consider it as a problem for the working field-naturalist of to-day 

 and to make various surmises and suggestions about its relations 

 to existing habitats and climatic conditions. The two points of 

 view are in no way antagonistic, the one is the complement of the 

 other. With further knowledge it Avill be possible some day to 

 weave them together into a coherent whole, but just now I can do 

 no more than to give some illustrations which indicate that the 

 matter is worth more detailed and intensive examination. 



(1) Some species (e.g. Hyalinia alliaria, Arion ater, Pyramidula 

 rotundata, Cochlicopa lubrica, lAmncea peregra) are found commonly 

 throughout, showing that they can tolerate the climate everywhere 

 and that habitats suitable for them are to be found throughout the 

 islands. 



(2) Some species {Limax cinereoniger, L. tenellus) occur from the 

 north of Scotland to the south of England, but are not common ; 

 they have no geographical distribution, except that L. tenellus is 

 not found in Ireland, and their occurrence seetns to be determined 

 by the existence of suitable habitats in the shape of ancient wood- 

 lands. Similarly Succinea oblonga and Vertigo minutissima are rare 

 species with a range from the south of England to the middle of 

 Scotland, whose occurrence probably depends on som.e as yet un- 

 defined quality of their habitats. 



(3) Hygromia fiisca, Acanihinula lamellata, Pupa anglica, and 

 Margaritana margaritifera are definitely northern and western, 

 being either absent or rare in the south-east. It seems fairly certain 

 that margaritifera can live only in waters containing little lime and 

 the rivers of the south-east are all calcareous. 



(4) In contrast with these we have a larger number of species 

 which are south-eastern in distribution, (a) such as Theba cantiana, 

 Helicigona lapicida, Azeca tridens, Pupa secale, Clausilia rolphii, 



