262 DREISSENAD^E. 



In 1825, in a List of Shells not taken notice of 

 by Lamarck (Ann. Phil. 1825), I stated that this 

 shell would " perhaps form a genus distinct from 

 Mytilvs, and peculiar for its fresh-water habitation," 

 and added that, " like Mollusca of that station, the 

 animal can live for a long time out of water. I have 

 kept one for three weeks, when it was still healthy. 

 It is found in the Commercial Docks, where it most 

 likely has been introduced with timber from the 

 Volga." 



I am now confirmed in the idea that they were 

 introduced in this way, as a friend has informed me 

 that he has seen them sticking to the logs of Baltic 

 timber before it was 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 logs. 



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 Journ. i. 584.), first recorded it as 

 naturalised in the Commercial Docks, where he 

 observed that it had probably been brought with 

 the timber ; it has since been widely extending it- 

 self, and is now to be found in most of the docks 

 connected with the Thames. In 1834, Mr. Stark 

 communicated to the Wernerian Society the dis- 

 covery of this species in the Union Canal, near 

 Edinburgh ; and, in 1836, the Rev. M. J. Berkeley, 

 the eminent cryptogamic botanist, and Mr. J. Streat- 

 field, discovered it on the piers of the bridge which 

 crosses the Nen at Fotheringay, and again, a little 



