244 JOURNAL OF CONCHOLOGY, VOL. I5, NO. 8, JANUARY, I918. 



Storms lias^ confirmed the exiguous results of fishing. Closed and 

 running ponds yield about the same total of species, though a more 

 detailed analysis shows that running ponds contain on the average 

 3'7 species, closed ponds 2 '3 sorts, or including the snailless 

 ponds only 17 ; the four running ponds in which the flow is 

 perennial average as many as 7 '5. The largest number of species in 

 any running pond is eleven (^peregra, palustris, stagnalts, albus, vortex, 

 contorttis, fontanus, jenkinsi, corneiim, subtruncatum, jnilhmi) ; the 

 most prolific closed ponds yield eight and nine sorts respectively 

 {^peregra, fofitanns, nauiileus, lacustris, cornei/m, lacustre and rnilhwi 

 in both, with palustris in one and casertanwn and subtrimcatiwi in 

 the other). 



It appears, therefore, as if one could arrange the habitats in an 

 ascending order of general suitability — closed ponds, running ponds, 

 river. And the question immediately arises, whether this may not 

 be just the order in which these different places are the more likely 

 to receive additions to their fauna. If one assumes that the distri- 

 buting agents of freshwater mollusca act in such a way that the dis- 

 persal they effect is equivalent to promiscuous projection from the 

 sky, it is evident that a river valley with secondary flood dispersal 

 offers a larger collecting ground than the basin of a pond, and that 

 a running pond is in the same way more apt to accumulate species 

 than a closed pond. But it is more reasonal)le to assume that dis- 

 persal by birds, beetles, frogs and what not is not at random, but in 

 definite relation to units of water, and not directly proportional to 

 their size. In any case the present comparison is not between one 

 river and one pond, but between a river and more than a hundred 

 ponds, which as recipients of indiscriminate dispersions are probably 

 collectively of the same order of magnitude as the Colne. Provision- 

 ally, therefore, I would reject the possibilities of dispersion as being 

 the complete or even the main explanation of the phenomena under 

 discussion, a conclusion which receives I think substantial support 

 from a consideration of the facts relating to individual species. 



L. auricularia, for example, is restricted to the river in the north 

 and the lakes in the south ; L. peregra occurs in both places and in 

 51 of the 108 ponds which contain any snails. The eggs, young, 

 adults and habits of the two species are so similar that it is hardly 

 possible to believe that suitability for dispersion is the explanation of 

 the difference in their distribution rather than suitability of the locus 

 into which they happen to be dispersed. P. cotitortus similarly is 

 found in the river, in two permanently running ponds (in connection 

 with one another) and in one stream, but in no closed ponds ; by 

 contrast nauiileus is the most frequent species in closed ponds, occur- 



