242 JOURNAL OF CONCHOLOGY, VOL. 15, NO. 8, JANUARY, 1918. 



time in trying to define our terms. "Lake," "pond," "pool," "stream," 

 " ditch," are at once elusive and precise ; at any rate meticulous de- 

 limitation will demand a fresh terminology. " Lake " connotes a 

 substantial size, a depth in parts beyond the limit of most water 

 plants, the possibilities of wave action ; " ponds '' are smaller — the 

 present series mostly about 400 square yards in area or less, shallower, 

 more fluctuating in volume ; " streams " are, in parts at any rate, 

 dominated by a rapid current, but physically they are not homo- 

 geneous ; " ditches " by their existence postulate a flow of water at 

 some time or other ; of " tarns," " meres," " loughs " there are no 

 local representatives. 



It may be objected to the whole enquiry that it is not possible to 

 find out accurately what is the molluscan content of any body of 

 water, however small. The Aldenham ponds have each been ex- 

 amined at least twice and the variations in the results have been 

 relatively trivial. There is no doubt that the snail population of 

 ponds varies both by accretion and by loss, and any investigation for 

 the present purpose must therefore have a time limit, so that it is 

 hardly possible to make the infinitely frequent examinations which 

 might solve the question as to whether " the snail is not there " and 

 " I cannot find the snail there " are as nearly synonymous as we 

 flatter ourselves. On the whole I imagine that the two expressions 

 are not very far from being equivalent, with the proviso that search 

 has been made at suitable seasons and in a proper temper of body 

 and mind. At the worst, the present facts may be taken as compar- 

 able among themselves ; with the same worker a negative result in 

 one pond cannot be worth very much more or less than a negative 

 result in another. But from these optimistic conclusions I should be 

 inclined to exclude the smaller species of Pisidium, which are apt to 

 live in very localised areas in relation to the absence on the bottom 

 of the homogeneity which characterises the main bulk of any body 

 of water. 



The results obtained in this way are briefly summarised in the fol- 

 lowing table. The river, the lakes, the streams and all the running 

 ponds contain snails of one kind or another ; of the 113 closed ponds 

 27 have yielded none, and for purposes of comparison it is probably 

 best to consider only the remaining 86 which are demonstrably as 

 well as presumptively compatible with molluscan life. 



The different localities evidently diff'er a good deal both in their 

 richness in molluscan life and in the kinds which are prevalent. The 

 river with its annexes yields as many as 26 different species, of which 

 six are small Pisidia ; this is not due to a summation of habitats of 

 obviously varying qualities ranging from the rapids of the river on the 



