DISPERSAL BY MAN. 211 



the Commercial Docks, in the Thames, by James 

 Bryant, Esq., who was in the habit of using the animal 

 as a bait for perch-fishing, and this is the first mention 

 of the existence in Britain of the now common and 

 widely distributed D. polyinorpha. In a letter accom- 

 panying the shells, Mr. Sowerby suggested that the 

 species, which was known in the Danube and in the 

 rivers of Russia, had probably been imported into this 

 country with timber, and in the following year Mr. Gray 

 expressed a similar opinion, believing the creature to 

 have been brought with timber from the Volga, and 

 further pointing out that it was able to live for a long 

 time out of water, he having thus kept one in health for 

 three weeks. As some molluscs attach them.selves to 

 the bottoms of ships. Sir C. Lyell was led to suggest 

 that the present species might have been introduced in 

 this way, and it was explained by Mr. Garner that by 

 keeping the valves constantly closed it was perhaps able 

 to survive immersion in salt-water during the voyage. 

 It is much more probable, however, that the creature 

 was imported with timber as suggested by Sowerby and 

 Gray, and this was afterwards admitted by Lyell him- 

 self. A friend of Gray's is said to have seen the 

 animals sticking to Baltic timber, which had not been 

 unloaded, and was still in the ship's hold.^ 



• "Trans. Lin. Soc," xiv. (1825), p. 585 ; "Ann. Phil.,'' (n.s.), ix. 

 (1825), p. 226; "Zool. Journ.," i. (1825), p. 584 ; J. E. Gray, "Ann. 

 Phil.," (n.s.), ix. (1825), p. 139; Lyell, " Principles," ed. 9,(1853), 

 p. 652; R. Garner, Charlesworth's "Mag. Nat. Hist," (n.s.), iii. 

 (1839), pp. 302-3 ; Gray's " Turton,'' 1857, p. 262. 



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