166 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



tlie extreme soiith-west of England. Our ignorance of the breeding 

 habits of many of our common species makes the inquiry difficult. 



(6) The influence of calcareous soil cannot be summarily dealt 

 with on a small scale map. Quite small areas, such as the bosses 

 of chalk that come through the overlying strata in Norfolk or the 

 patches of cornstone in the old red sandstone of Herefordshire, 

 are imperceptible without local knowledge, but have their char- 

 acteristic influence on the fauna. There seem to be only three 

 species with absolute relations : Zonitoides excavatus is never found 

 on calcareous soil, Cyclostoma elegans is never found away from it, 

 Margaritana margaritifera occurs only in soft water. Other species, 

 e.g. Pupa secale (which extends northwards to Cumberland) or 

 Clausilia rolpJiii certainly show a very marked association with 

 chalk and limestone. But in the case of several south-eastern 

 species [Helix pomatia, Ena montana, Helicodonta ohvoluta) it is 

 difficult to say whether their distribution is determined by 

 geographical position or by the lime in the soil : the geological map 

 does not lend itself to the solution of the problem. The whole 

 of the area in which they occur might from the map be reasonably 

 supposed to be calcareous, and most of it actually is, but H. ohvoluta 

 certainly (and I think also the other two) is occasionally found 

 in places where neither the surface soil nor the vegetation is chalky. 

 These species, then, may belong to the same distributional group as 

 Helicigona lapicida, Theba cantiana, Helicella heripensis { = gigaxii), 

 which are southern and eastern in their range, and therefore must live 

 mostly on calcareous land : but they all extend sufficiently to the north 

 and west to afford so many exceptions that one must regard their 

 relation to limey strata as a coincidence. As regards water sj)ecies, 

 rivers represent the strata of their origin and the hardness or softness 

 of their water may be deduced in most cases from the geological 

 map. But ponds of soft water occur in calcareous districts, and 

 springs and streams of very hard water may occur in non-calcareous 

 areas : for these smaller waters local knowledge is necessary. 



(7) Water snails show distributions which are analogous to 

 those of land species. Limncea glabra and Planorbis glaber are 

 rare in the south of England, and Segmentina nitida seems to be 

 mainly a south-eastern species. There is some evidence from 

 aquarium observations that L. glabra dislikes water well warmed in 

 a summer sun. In a broader way Planorbis corneus. Bithinia 

 leachii, Paludina vivipara, P. contecta, Dreissensia polymorpha,^ 

 TJnio pictorum, Unio tmnidus, and S2:)ha}rium rivicola are south- 

 eastern, but their distribution has perhaps been so much influenced 

 by canals that their present cannot be taken to represent indubitably 

 their normal range. Practically all our canals were made between 



^ This species was re-introduced into this country in 1824 : it is, therefore, 

 hardly in the same category as the others. 



