BOYCOTT : (ECOLOGICAL NOTES. 167 



1760 and 1810, and they joined up the various river basins from 

 Westmorland and Yorkshire to Wiltshire and Somerset, and from 

 Norfolk to Montgomery and Hereford (PL V). The possibility of 

 spread by their agency is particularly strong in Dreissensia 

 polymorpha and Spcerium ovale. Just as the provision of these very 

 favourable habitats may well have helped a number of species to 

 spread over central and southern England, so the rarity of suitable 

 places (ponds, slow rivers, etc.) in western Wales and Devon and 

 Cornwall may explain the absence of Planorhis corneus, PI. complanatus, 

 LimncBa stagnalis, and L. auricularia from these areas. The distribu- 

 tion of the Unionidae maybe related to the occurrence of appropriate 

 fish on which to pass their parasitic phase, but I do not know of 

 any definite indication in this direction for our British species. 



The problem of the distribution of our British land and fresh- 

 water moUusca seems therefore to be divisible into three groups of 

 questions : — has the species ever had a chance of getting to the 

 place 1 — is the climate suitable ? — is there a suitable habitat ? In 

 my own parish, for example, Margaritana margaritifera does not 

 occur because the river, suitable in being not stagnant and in 

 containing trout which are known to be a satisfactory host for its 

 glochidia, has hard water ; Limax tenellus does not occur because 

 tliere are no ancient woods, its abundance in other woods not far 

 distant testifying to the suitability of the climate ; Hygromia fusca 

 is absent because the climate of southern Hertfordshire is too dry 

 for it. Limax tenellus may be presumed to be absent from Ireland 

 because it has never been able to get there ; while Hyalinia lucicla, 

 often a garden species, has been transported and has become 

 common in most parts of that country : but it may well be doubted 

 whether the climate is suitable for Helix pomaiia or Helicodonta 

 ohvoluta. Each species raises questions of great complexity, and 

 I have been able only to indicate in the briefest outline some of 

 the ways in which these may be approached. 



GECOLOGICAL NOTES. 

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



(Continued from p. 130.) 

 Read 13th May, 1921. 



4. The Habitats of Limax maximus and L. ciNHBEONiaEB. 



L. mnximus lives happily in gardens and cultivated ground : 

 it also flourishes in vvild places, expecially woods. L. cinereoniger, 

 on the other hand, is ordinarily found only in wild places, and it seems 

 probable that its occurrence in a wood may be taken as evidence 

 that the place is ancient forest : it is intolerant of civilization and 

 cultivation. The point to which 1 particularly desire to draw 

 attention is that the two species are seldom found together, from 

 which it would follow that woods which are suitable for cinereonioer 



