OBSERVATIONS ON MOLLUSKS. 



FEW things are more remarkable in Natural His- 

 tory than the sudden appearance of species, in great 

 plenty, in places in which they had been previously 

 unknown. This has often been observed among 

 insects, but such an occurrence is not confined to 

 that class of animals. It happens not unfrequently 

 with animals of other classes. I have twice espe- 

 cially had my attention called to this circumstance in 

 the case of the fresh- water mollusks. The first 

 instance occurred in 1822. During the spring and 

 summer of that year, some small pits in this parish, 

 the bottom of which consists of a gravelly clay, and 

 which are generally full of water, but sometimes dry, 

 or at least many of them, during the warmer months, 

 swarmed with the Limneus glutinosus of Draparnaud * 

 to such an extent that the shells might be scooped 

 out by handfuls : in some places, if a bucket had 

 been lowered into the water, it might have been 

 drawn up half full with them. Many other species 



* Amphipeplea glutinosa, Nills. Gray's edit, of Turton's Man. 

 of Brit. Land and Fresh-water Shells, p. 243, tab. ix. fig. 103. 



