86 THE DISPERSAL OF SHELLS. 



manner.^ I cannot think, however, that the operculate 

 pond-snails often cling in this way, and, as already 

 mentioned, those with which we are familiar in this 

 country are known to be very generally absent 

 from perfectly isolated waters.^ In July, 1891, I 

 experimented, with grass-stems, on a good number of 

 specimens of Paludina vivipara in the Lea marshes at 

 Tottenham, and it seemed hardly likely that animals 

 chancing to insert their toes into the mouths of these 

 shells would often be entrapped, for the opercula were 

 not firmly closed with sufficient suddenness. Most of 

 the specimens which were induced to hold on to the 

 inserted stems dropped after a minute or two ; one, 

 however, did not fall until I had carried it, suspended 

 upon the stem, for more than half an hour, but a sharp 

 jerk would probably have caused it to drop almost 

 immediately. 



The fresh-water limpets {Ancylus), inoperculate 

 univalves which generally adhere to stones and water- 

 plants in ponds and rivulets, were specially mentioned 

 by Reeve as having very limited facilities for migration.' 

 But they sometimes ride upon the backs of large 

 flying water-beetles I The taking of a great water- 

 beetle (Dytiscus) with an Ancylus firmly adhering to it 



» F. W. T., " Humble-bee trapped by snail," Field, Ixv. (1885), 

 843. 



' See Clement Reid, on the " Natural history of isolated ponds," 

 "Trans. Norfolk and Norwich Nat. Soc," v. (1892), 279. 



» Lovell Reeve, " Land and Fresh-water Mollusks," 1863, 

 p. 255. 



