THE LAND AND FRESHWATER MOLLUSCA OF DORSETSHIRE.. 7? 



when rambling among the water plants. Mr. Wallace speaks 

 of a small shell (Ancylus) having been found attached to a 

 water-beetle. Land shells may possibly be carried down by 

 floods to the sea, and safely deposited on the neighbouring 

 shore. Mr. Darwin shews they have a remarkable retention of 

 life, and can survive after many days immersion by the 

 formation of a membraneous diaphragm at the mouth of the 

 shell. He cites a case, in which out of one hundred shells 

 immersed for a fortnight in the sea, twenty-seven survived ; 

 drift-wood, too, will preserve the little animals in its chinks and 

 rugosities. Young Unios and Anodons will occasionally affix 

 themselves to the lips and fins of fish. The food of the bivalves 

 consists principally of Infusoria, Desmidiae, Diatomacese, &c., 

 conveyed to them by the currents which are formed by the 

 marginal ciliary apparatus of the mantle ; Sphserium is some- 

 times found embedded in the waterlogged flesh of drowned 

 animals. Land gasteropods are for the most part herbivorous, 

 but some will become flesh-feeders under special circumstances. 

 During the winter months the freshwater mollusca will bury 

 themselves in the mud of ponds and rivers ; the land mollusca, in 

 the ground, or beneath moss or dead leaves ; some will secrete a 

 covering to the peristome, and form a false operculum, like the 

 Helix pomatia ; others will seal the aperture of their shells with 

 a thin filament, as does the Helix aspersa. 



The land and freshwater shells of Great Britain are identical 

 with those of the Continent ; the Channel seperating these 

 two portions of the same zoological province. Of the entire list 

 of British Land and Freshwater Shells, only one is exclusively 

 British (Zonites excavatus), and this Mr. Gwyn Jeffreys believes to 

 be identical with the Helix vitrina of Ferussac, under its varietal 

 form, vitrina (or viridula, of Menke), in which case every one of 

 our British Land and Freshwater Shells are represented on the 

 other side of the Channel. 



In the arrangement, and nomenclature of the list I have 

 followed Mr. Gwyn Jeffreys, whose valuable work, "British 



