8G NOTES ON LAND AND FRESH-WATER MOLLUSCA. 



near Thirsk, in search of this shell, not knowing its peculiarity. When I 

 arrived at the foot of the mountain, I could find no traces of it, but thought 

 when I got to the top, I should be fully repaid; there again I was disappointed, 

 and had the mortification of only meeting with a couple of specimens, com- 

 pletely blanched by exposure to the sun and weather; and was obliged to 

 return without obtaining the object of ray journey; the only consolation I 

 received being a few mosses and lichens, which were new to me. 



I was somewhat disappointed a second time, when on a tour through Wen- 

 sleydale, at finding but few specimens of this shell, which had been collected 

 in great abundance near Aysgarth, a few months before, by a young friend, 

 who was with me at the time, and had no doubt but that we should meet 

 with it again; but though there had been a considerable shower of rain a 

 few hours before we visited their habitat, it was only after a laborious search 

 that we found a few specimens; whereas had there been a continuous rain 

 for some time, we should have found them crawling in all directions over the 

 low walls which abound there, in place of hawthorn hedges; for it was during 

 a wet season that my friend had found them. 



The same fact is true of Glausilia duhia^ which is found chiefly and most 

 abundantly in Limestone districts, though I have not found it in this neigh- 

 bourhood, where Magnesian Ijimestone abounds; but we have Helix Lapicida 

 in considerable numbers. The first time I met with Glausilia dubia was close 

 by Houghton-le- Spring, on the Sunderland road, where it is cut through the 

 Limestone Cliff". It was extremely abundant, together with Bulirnus obscums, 

 as a quantity of rain had fallen the previous night. But once when I sent 

 to a friend in the neighbourhood, to gather me a few more specimens, it 

 was with difficulty he could find a few stragglers. Zonites umbilicatus may 

 be ranked among those enumerated above, as it is essentially a Limestone 

 Shell. 



Though I have mentioned these particularly, I would not deny that all 

 Land Molluscs are eff"ected by the rain; we need only go a few yards from 

 our own doors, after a wet night, to feel considerable difficulty in avoiding 

 the destruction of numerous Limaces, Arions, etc., which are crawling about 

 on the paths. But I consider that those I have mentioned are peculiarly 

 sensitive to moisture. Probably it is owing to the almost utter absence of 

 moisture, in any other than damp weather, on the rocks and walls, which 

 the pupa, etc., inhabit, which causes them to seek crevices, and to bury them- 

 selves amongst the stones and in the soil. 



These remarks have been made chiefly because many who collect Shells, 

 are often disappointed at not finding those they are looking for in any par- 

 ticular locality, though they have been told of their abundance there; and 

 that they may know that the best time for making a crusade against such, 

 is during or soon after wet weather. 



Another circumstance has very much interested me, that is, the gregarious 

 nature, if we may so express it, of several species, which literally swarm in 



