NOTES OX LAND AND FRESH-WATER MOLLUSCA. 87 



some places, and yet extend to no very great distance. Among such may 

 be numbered Helix virgata, ericetorum, and Cantiana, particularly virgata, 

 which I first found near Middlesbro' in the greatest abundance, but confined, 

 as far as I could discover, to comparatively small areas. 



This might be owing to a difference in the soil or formation, did not ob- 

 servation prove the contrary. When at Thirsk once, I observed this fact 

 with regard to Cantiana, which was pretty common a mile or two out of 

 the town, but confined to a small space; and if I had not been directed pretty 

 clearly to the spot, I should have had some diflBculty in finding it. Azeca 

 tridens seems to be another of the same class. This, though by no means 

 an abundant shell, I have found occasionally in considerable numbers near 

 Ay ton; and as I always found the above rule hold good, I have come to 

 the conclusion that, generally, when one is found, a colony is not far distant^ 



With regard to Water Shells, I have frequently noted that one or two 

 species abound to the exclusion of others. This, I have thought, is owing 

 to one having got the upper hand, and preying upon other species, or at any 

 rate destroying them. This is very noticeable in the Limnoeus tribe. The first 

 specimens of Limnceus Stagnalis, which I remember to have seen, were from 

 some ponds near Stockton, collected by my friend, George Dixon, of Ay ton; 

 and, with the exception of Velletia lacustris, which was found riding on the 

 backs of the Stagnalis, there was scarcely another Shell in the pond, except, 

 perhaps, Cyclas cornea; whilst in other ponds in the neighbourhood there was 

 not a single specimen of Stagnalis, but abundance of Pereger. The only 

 ponds in which I have found Cyclas lacustris, have been almost destitute of 

 other species; in one near Ay ton they were very plentiful. Planarbis Icevis 

 is found in a small fishpond here, and though almost the whole district for two 

 or three miles round, and in some directions for five miles, has been diligently 

 explored, I am not aware that they have been found in any other pond in 

 the neighbourhood, though they are particularly abundant in the above-men- 

 tioned pond, but no other Shell is to be found along with them, except a 

 few L. pereger. In fact I might go on enumerating almost the whole of the 

 Fresh-water Shells, which have come under my own observation, as examples 

 of what I have stated. The Pisidium is a good example. If two pieces of 

 water in the same neighbourhood are found to contain Pisidium, it very 

 often turns out that the whole of those in the one are of a different species 

 from those in the other. — This, though not invariably the case, I have found 

 very frequently so. I have observed, too, that in the same running stream, 

 where more than one species of Mollusc abounds, they seem to congregate, 

 as it were, in colonies; but at the same time mixing more or less with each 

 other, from the outposts of one species coming in contact with those of another, 

 and from their being frequently carried along by the stream without exercising 

 any will of their own. 



A fruitful source of some of the smaller Shells, as Pupa, Vertigo, etc., 

 to the mere collector, is the debris which is brought down by various streams, 



VOL. IV. N 



