18 



sbells. It is gregarious, and frequeatly discovered at- 

 tached together in great numbers. 



This shell lives in both fresh and brackish water, and 

 is capable of living for a time in the salt ocean. Con- 

 chologists are thus enabled to account for its introduc- 

 tion into Great Britain, for it in all probability found 

 its way into this country clinging to the bottom of vessels. 

 Mr. Sowerby was the first to call public attention to this 

 shell, in 1824, having procured specimens from the 

 Commercial Docks of London. In 1834, Mr. Stark 

 discovered it in the Union Canal, near Edinburgh; and 

 the Rev. M. J. Berkeley, in 1836, found it in the 

 river Nen. 



It seldom leaves the substance to which it attaches 

 itself by its beard. Four specimens, which were found 

 clinging to a stone in the lake at Highfield House, 

 have not left it for two years ; during this period they 

 increased greatly in size, and gradually became more 

 marked with chocolate stripes, these markings becom- 

 ing darker iu colour. 



I can scarcely imagine so conspicuous a shell as 

 Dreissena polymorpha to have been an inhabitant of the 

 British Isles for more than one or two years prior to its 

 discovery by Mr. Sowerby, for it would hardly have 

 been overlooked by such conchologists as Montagu, 

 Donovon, Turton, Grey, and Leach ; and yet, if it has 

 been so recently introduced, it is marvellous how 

 rapidly it has become widely spread over our island. 

 I can easily conceive it being soon scattered through 

 our rivers and canals by the agency of vessels and boats, 

 but in what manner it has introduced itself into such 

 isolated lakes as those of Wollaton and Highfield 

 House it is diflScult to understand. 



Water fowl evidently feed upon the Dreissena, as, on 

 the banks of the lake at Wollaton, numerous dead 

 specimens were found lying about. 



