262 
DREISSENAD^. 
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 
Mytilns^ 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 Eev. 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 
