250 JOURiSIAL OP CONCHOLOGV, VOL. 1^, NO. 8, JANtJARV, I918. 



or J^. albus. The latter is a definite factor of more general applica- 

 tion to the superiority of running over still water. 



{c\ Lastly we have two species which are most frequent in still 

 water. Planorbis nautileus is in 40 of 86 closed ponds, in 7 of 22 

 running ponds (five of which are running only in the winter) and no- 

 where else ; Sphcerium lacusire occurs only in 16 closed ponds and 

 in two winter running ponds. Both species are so common that the 

 question of dispersion can be reasonably neglected as a preliminary, 

 though the possibility of a dispersive agent with a selective taste for 

 still water has to be noted. We seem in fact to have two species 

 which prefer loci more or less inimical to other moUusca. Similar 

 occurrences are sometimes attributed to the inability of the species 

 occupying generally unfavourable habitats to occupy more advantage- 

 ous positions owing to the competition of more powerful kinds ; the 

 weaker species are thus driven into the cecological slums. However 

 true this concepton may be in the case of plants and of animals whose 

 limiting factor of abundance is food, one must, I think, be cautious 

 of using it too freely. If we try to reduce the general idea to a 

 definite imagery of what is happening in the special case of the mol- 

 lusca we are considering — if we consider whetlier what we know of 

 the economy of the mollusca does not suggest that the prevalent 

 populations are as probably determined by an abundance of enemies 

 as by a deficiency of food — if we wonder why 26 species should be 

 able to flourish abundantly together in the large river backwater, and 

 should yet by something they think or do in drawing a line of quality 

 be able to exclude two further species belonging to genera already 

 represented— this idea of competitive seclusion seems unattractive. 

 Be it noted too that the evidence that lacusire and especially nautileus 

 do not occur in favourable locaUties is not really so good as the indi- 

 cations that they are found in unfavourable loci. If the former is 

 actually the fact, the natural surmise is that some relatively destruc- 

 tive agent, more probably biological than physical or chemical, is 

 prevalent in favourable places ; if the latter, it is clear that they are 

 adapted to withstand bad conditions in ways which with fuller informa- 

 tion are probably capable of analysis. The comparison oi P. amnicum, 

 S. corneum and .S. lacustre should be particularly instructive. 



Such in very brief outline is the topic which requires our curious 

 and careful consideration. Several groups of important habitats and 

 many species of interest have not been mentioned because they are 

 not represented locally. Peaty waters for example have special con- 

 ditions of fauna and flora; marshes, canals (of all habitats apparently 

 the richest) and natural lakes require attention. The rare species 

 {^A. glutinosa, F, giaber^ S. lineata, Sph. ovale) are of special interest ; 



