II.] INTRODUCTION. xliii 



lusca in places where they had not been previously 

 noticed is not unfrequent. Many a wonderful tale of a 

 "shower of snails" has helped to fill the pages of country 

 newspapers, arising out of the sudden appearance in a 

 limited area of Helix virgata vaidBulimus acutus, which 

 are abundant on most of our sandy downs and plains. 

 This has been sometimes caused by a mere change of 

 wind to the south-west, in consequence of which the air 

 has become charged with moisture, and tempted all the 

 snails to leave their retreat at the roots of grass during 

 the night, and to present themselves in the morning to 

 the eyes of astonished rustics. It is not, however, so 

 easy to account for some kinds of freshwater snails (e. g. 

 Limncea glutinosa) being found in the same spots so 

 abundantly some years, and scarcely at all in others, as 

 has been noticed by Mr. Bridgman of Norwich. This 

 is one of the minor, but numerous, problems in the pre- 

 sent branch of Natural History which still remains to be 

 worked out, and the solution of which will reward the 

 diligent and observant conchologist. 



Tenacity of life. Many of the Mollusca, as before 

 remarked, pass the winter in a state of torpidity, their 

 vital functions being apparently suspended during hiber- 

 nation. But some of the land-snails have been known 

 to live many years shut up in boxes and drawers, or 

 affixed to tablets as specimens. This capability of sub- 

 sisting for a long period without food is probably owing 

 to the snail being able to close its shell by an epiphragm, 

 which not only prevents the evaporation of its natural 

 moisture, but also produces a kind of protracted hiber- 

 nation. Miiller relates that some snails, from which he 

 had cut off their heads, lived more than a year in this 

 state without food, crawling about, and at the usual 

 time forming their winter epiphragms. Some marine 



