83 

 THE ZEBRA-MUSSEL (Dreissensia polymorpha Pallas). 



By R. STANDEN. 



(Read before the Society, January 13, 1904). 



Of all our British non-marine moUusca I consider the "Zebra-mussel" 

 second to none in the peculiar interest attaching to its rapid distribu- 

 tion and life-history. A brief resume of what is known on the subject 

 may be useful to those members who are not fully cognizant of it, and 

 will, I hope, give some additional interest to the special exhibition of 

 Dreissensia shewn at this meeting. 



During the later Tertiary times Dreissensia was well distributed over 

 Europe, and seems from some unknown cause to have died out, but 

 during the past two centuries it has partially regained its position by 

 migration from its original habitat, the Caspian Sea. It first appeared 

 in this country in 1824, and there is very conclusive evidence to hand 

 of its being a foreign importation — probably from Germany. 



With its general geographical history I do not propose to deal, and 

 shall only touch upon its local distribution in Lancashire and Cheshire, 

 as ascertained from various authors, local collections, and my own 

 experience. Those who desire fuller information may refer to the 

 appended list of the principal authorities I have consulted, and more 

 especially to Wallis Kew's admirable work on the "Dispersal of Shells" 

 ('93) ; this writer quotes exhaustively from many out-of-the-way 

 sources, and gives the authors' opinions as to the causes to which the 

 rapid dispersal of Dreissensia throughout this and other countries may 

 be attributed, and the ingenious theories propounded by the earlier 

 conchologists to account for it. The first Lancashire record I can 

 find is by Captain T. Brown ('44), who states that it was common in 

 the Bridgewater Canal. David Dyson ('50) found it existing in great 

 numbers in a reservoir at Beswick, covering the embankment stones 

 at about three feet deep. He was led to investigate the reservoir by 

 the discovery of the shells in the large old-fashioned stone pipes then 

 used to convey the water to Manchester. It was found that these 

 pipes, specimens of which may now be seen placed as curiosities in 

 most of our public parks, had become so choked with the shells as to 

 seriously diminish the volume of water and interfere with the supply, 

 and it was almost an impossibility to clean them out. In spite of the 

 absolute darkness in which these shells lived and throve, they were 

 unusually well marked and brilliantly coloured. Then we come to 

 the late Mr. John Hardy's record ('65):— "Old brick pits, Longford; 

 and canals at Reddish and Bolton." The late Mr. Thomas Rogers 

 found it, about the same date, very common in the Bardsley Canal; 



