2^6 JOURNAL OF CONCHOLOGY, VOL. Ij, NO. S, JANUARY, tgiS. 



suffice. Our ponds aie not altogether without fish ; carp occur in 

 three closed ponds, sticklebacks (6^. aculeatus dind piitigitius) in seven 

 closed and nme running ponds and likely enough in others that I 

 have missed. Twenty fishy ponds out of 135 is rather a thin distri- 

 bution of an essential factor, and the absence of Anodonta in the 

 ponds may be due to the failure of the necessary concurrence of 

 chances. I suspect, however, that these large animals prefer the 

 larger bodies of water on other grounds than homoeopathic magic. 



{p). Bithinia and I'alvata with Fiatiorbis coriieus and F. amnicum 

 are restricted to the river area. As regards the operculate species, it 

 is possible that their capacity for clinging to translated objects dimin- 

 ishes their chances of free dispersion ; it may well be imagined that a 

 young Bithinia, finding itself on the historical duck's foot, would he 

 frightened enough to close its operculum without waiting for the new 

 pond of which unconscious memory might warn it. It is on the 

 other hand easy to exaggerate — or mhiimise — the difficulties of travel 

 when we have no real knowledge of how it is in fact accomplished ; 

 and it must not be forgotten that with the operculum is correlated a 

 branchial rather than a pulmonary respiration.^ 



(y). The remaining three sorts occur in the river and in other 

 running water : Ancylus in the brook, P. fontinalis in a branch 

 stream, PI. contortus in another branch stream and in two communi- 

 cating permanently running ponds. There is no obvious explanation 

 of this distribution ■. and why these species, together with L. auricn- 

 laria, PL. corneus, and P. ajimicum^ find in ponds an uncongenial set 

 of circumstances is apparently the same problem as why PL complati- 

 ' aius and especially perhaps Pi. vortex prefer running ponds to closed 

 ponds, and why in general running water provides more prolific loci 

 than closed ponds. So also as there is a continuous gradation from 

 quick streams to still water, which for convenience we have to divide 

 into aroitrary groups, there seems to be a series of snails varying from 

 Ancyius Jiuviatiiis which will only live in definitely running water, 

 through e.g.^ Pianorbis corneus, which at any rate very much prefers 

 moving water, to sorts like PL alb us or L. perep-a or A. lacustris, 

 which seem more or less indifferent. Whether any species actually 

 prefer stagnant water in the ordinary sense is discussed below. 



The question is perhaps best approached in the form of an enquiry 

 why small closed ponds afford an indifferently good habitat. It is 

 not possible here to discuss fully the many considerations which may 

 be germane ; a brief outline of the more obvious factors is all I can 

 attempt. 



I Our district does not supply the evidence that these species are not excluded from the 

 ponds and lakes simply because the water is soft and the river water hard ; £. tcniaculata 

 and F. aiiuiicuiii iive in the very soft water of the Herefordshire Wye ; P. amnicum, B. ten. 

 tacutata and /'. pisciiiaUs in the ManctiesCer city supply. 



