80 The Mollusca of Wiltshire. 



in the Canal at Trowbridge, Salisbury (Vize) ; Dr. Maton also re- 

 corded it from Salisbury ; Great Bedwyn (Townsend) ; canal at 

 Devizes (Heginbothom) ; common in stream, Mere (E. Baker). 



Dreissensia polymorpha (Pallas). — Apparently confined to 

 the Kennet and Avon Canal. Very abundant near Trowbridge in 

 the canal, and reservoirs and canal at Devizes (Vize) ; Devizes 

 (Miss Cunnington and Heginbothom). 



Is any thing known of the introduction of this species into the 

 Kennet and Avon Canal ? It is supposed to have been imported 

 into this country in or about the year 1824. Within twenty years 

 of that date it was reported from two counties in Scotland and 

 thirteen in England. It was unknown to the Bev. J. E. Vize, who 

 contributed a list of the Land and Freshwater Shells of Wilts, in 

 1863, to vol. ix. of the Wilts Arch. Mag., or, at least, he makes no 

 reference to it. In the 1889 Census List of British Non-Marine 

 Mollusca it is recorded from twenty-one counties and vice-counties 

 in England and two in Scotland. In the 1902 Census it is given 

 under twenty-five English — including S. Devon, N. Somerset, 

 Surrey, and Middlesex — and four Scotch. Wilts must now be 

 added to the list, and specimens have been sent to the Society's 

 Becorder. There are no records from Wales and Ireland. " The 

 Dreissena is perhaps better fitted for dissemination by man and 

 subsequent establishment than any other freshwater shell ; tenacity 

 of life, unusually rapid propagation, the faculty of becoming at- 

 tached by a strong byssus to extraneous substances, and the power 

 of adapting itself to strange and altogether artificial surroundings 

 have combined to make it one of the most successful molluscan 

 colonists in the world " (H. Wallis Kew, in " Dispersal of Shells," 

 p. 219). 



Unio pictorum (Linne). — Apparently a local species. At the 

 present time it has been recorded only from the Kennet and Avon 

 Canal and its reservoirs at Devizes (Vize and Heginbothom). 



Unio tumidus (Retzius). — Becorded from the northern part 

 of the country only ; has not, as yet, been observed south of the 

 Kennet and Avon Canal. Canal at Devizes (Miss Cunnington and 

 Vize); canal at Swindon (Cockerell) ; canal atWantage(Bromehead). 



