210 THE DISPERSAL OF SHELLS. 



than forty years previously/ It has been further 

 suggested that shells regarded as pertaining to a form 

 of Planorbis glaber Jeff., found associated with P. 

 dilatatus in a water-lodge at Burnley, may be Say's P. 

 parvus also introduced from America ; P. glaber, how- 

 ever — often regarded as identical with P. parvus — 

 though local, ranges widely in Europe, extending in this 

 country " from the Shetland Isles to Land's End," and 

 its right to rank as a native is not doubted/ 



A short account of what is known of D.polyuiorpJia and 

 P. dilatatus in Britain, and of the manner in which they 

 were probably introduced, may be of interest ; and it is 

 worth while, perhaps, to add a note on an exotic pond- 

 snail, Physa acuta, which, though not referred to in the 

 Conchological Society's list, is known to live, under 

 artificial conditions, in two localities in England, and 

 one in Scotland, and has in all probability been intro- 

 duced unintentionally by man. 



Dreissena polymorpha Pall. On 2nd November, 

 1824, the Linnean Society, as appears by an extract 

 from their minute-book, received from Mr. J. De C. 

 Sowerby, specimens of a fresh-water shell, " probably 

 the Mytilus polyuiorphus Gmel., 3363," which had been 

 found in abundance, attached to shells and timber, in 



• J. G. Jeffreys, "Ann. and Mag. Nat. Hist.," (4), iv. (1869), pp. 

 341-2 ; and see also W. C. Hey," Journ. of Conch.," iii. (1882), p. 

 271 ; T. Rogers, "Journ. of Conch.,"' v. ([887), p. 220 ; R. Standen, 

 "NaturaHst," 1887, p. 157. 



' T. Rogers, " Journ. of Conch.," v. (1887), p. 219; Jeffreys, 

 "British Conchology," i. (1862), p. 86; Rimmer, "Land and 

 Freshwater Shells," 1880, p. 40. 



