DREISSENAD.E. 303 



informed me that he has seen them sticking to the 

 logs of Baltic timber before they were unloaded from 

 the ship. (See Wiegmann, Arch. 1838.) In the dock 

 they attach themselves to stones, Uniones, Anodons, 

 and the walls of the docks, as well as to the loss. 



This species illustrates how rapidly molluscous 

 animals may become naturalised, and spread over a 

 great extent of country ; for Mr. J. de C. Sowerby, 

 in 1825 (Zool. Joitrn. i. 584.), first recorded it as na- 

 turalised in the Commercial Docks, where he observed 

 that it had probably been brought with the timber : it 

 has since been widely extending itself, and is now to be 

 found in most of the docks communicating with the 

 Thames. In 1834, Mr. Stark communicated to the 

 Wernerian Society the discovery of this species in the 

 Union Canal, near Edinburgh ; and in 1836, the Rev» 

 M. J. Berkeley, the eminent cryptogamic botanist, dis- 

 covered it, with Mr. J. Streatfield, on the piers of the 

 bridge which crosses the Nen at Fotheringay ; and 

 again a little higher up the same river, on stones of a 

 small overfall at Tansor : he believes they were intro- 

 duced from Wisbeach on timber since 1828. 



It has been naturalised into Holland and on the 

 Rhine. It is also found with tertiary fossils in 

 Transylvania, Moravia, and near Vienna. 



Mr. Lyell (GeoL), not being aware that these ani- 

 mals had the power of living a long time out of water, 

 and that they were most probably brought in the 

 holds of ships with the Baltic timber, and thus in- 

 troduced into our docks, where the timber is unloaded, 

 believes that the animals were introduced attached to 

 the bottom of Baltic ships, and thus obliged to pass 

 through the sea, before being again brought to their 



