130 JOURNAL OF CONCHOLOGY, VOL. l6, NO. 4, AUGUST, I92O. 



not be said to be very abundant. I am of opinion that the species 

 is here a waning one. From the ditches of the Morfa, with one 

 exception, H. ne?noralis has apparently disappeared. 



There appear to be two reasons for this : — Firstly, the system of 

 drainage, which has rendered the area dry and suitable for building 

 land ; secondly, the absence of sheltering hedgerows. In a few years 

 time the whole of the Morfa will become streets and houses, and the 

 remains of the molluscan fauna will have vanished. Across the 

 southern end of the Morfa is the main drain, once, no doubt, a clean 

 freshwater stream, but this appears now to be little better than an 

 open sewer, and devoid of molluscan life. Now, H. virgata and H. 

 barbara may be truthfully said " to swarm," for in many places they 

 could be scraped together in handfuls. 



In the South of England H. hortetisis is so abundant at times and 

 ascends rails and posts by roadsides in such prodigious numbers 

 that it is quite easy to fill a pint tin in a few minutes by merely 

 scraping the shells into the tin. The hedgerows, too, at times seem 

 full of snails, and great numbers can be collected in a short time 

 from beneath the bushes if beaten with a stick. Country people send 

 out their children in rainy weather to collect snails to feed ducks 

 and fowls, and it is no uncommon thing for a couple of gallons to be 

 collected in the space of half-an-hour. 



What are the causes of the mysteriously sudden disappearance, and 

 often equally rapid re-appearance, of many species of snails ? 



Frogs are said by rustics to come in showers, and no doubt the 

 myriads of H. virgata, etc., which suddenly appear on the downs 

 after rain are also believed by them to fall from the clouds. Perhaps 

 we may look for a satisfactory* answer to the question in the climatic 

 conditions at the time of our search. 



Suppose a period of droughty weather has obtained for some con- 

 siderable time, and a sudden change to rainy weather takes place ; if 

 the temperature be fairly high (say 60 deg. to 65 deg.) this, together 

 with a humid air, will bring out some species from their hiding places 

 in quick time ; but should the rain be accompanied with a cold east, 

 north, or north-east wind, few snails will be found, except, perhaps, 

 the few hardy ones, so that the collector who goes out under the 

 latter conditions will most likely return empty-handed from a spot 

 where, under more favourable weather, he would have secured speci- 

 mens in abundance. Humidity and a fairly high temperature are the 

 best possible conditions for collecting land mollusca. 



There is also another feasible explanation. After a period of 

 activity, during or immediately following rainy weather, snails usually 



