362 Naturalization of Dreissena polymorpha. 



gazine, vol. ix. p. 573. Among the various names which have 

 been proposed for this genus, the term Dreissena is the least 

 to be commended, being chosen in compliment to an indivi- 

 dual, instead of expressing any fact with regard to the genus 

 itself. Nevertheless, the term Dreissena has the clear right 

 of priority in point of date, and ought therefore to be perma- 

 nently adopted. 



Mr. Sowerby first noticed the introduction of this species 

 into Britain, in a communication to the Linnean Society, dat- 

 ed Nov. 2nd, 1824; (see Lin. Trans, vol., xiv. p. 585). He 

 states it to have been introduced into the Commercial Docks, 

 Rotherhithe, whither it had probably been brought attached 

 to Baltic timber. On visiting these docks a few months ago, 

 I observed multitudes of these shells in the depot for bonded 

 timber. It appeared that at least one generation had passed 

 away since 1824, for myriads of the dead shells were scatter- 

 ed over the bottom of the water, and served as points of at- 

 tachment for the byssi of the living. 



This shell was next noticed in the Union Canal, near Edin- 

 burgh, by Mr. Stark, in 1834. In November, 1836, the Rev. 

 M. J. Berkeley communicated to this Magazine his discovery 

 of the Dreissena in the river Nen, in Northamptonshire. I 

 have further to add the following instances of its occurrence, 

 which have come to my own knowledge. 



From the year 1828 to 1834 inclusive, I was frequently in 

 the habit of conchologizing in the Avon, near Evesham, dur- 

 ing which period, if the Dreissena polymorpha had inhabited 

 that river, I could scarcely have failed in detecting it. Not 

 the slightest vestige of it however occurred to me during that 

 time. An interval of two years elapsed, in which I was ab- 

 sent from England, and in the beginning of 1837, I was 

 much surprized at finding several specimens of this shell a- 

 mong the refuse on the banks of the river. On further search 

 I found that the Dreissena had become completely establish- 

 ed on the beds of gravel in the river ; and in the course of an 

 hour I collected several hundred full-grown specimens. There 

 is therefore clear evidence of the recent introduction of this 

 mollusc into the Avon, and of the rapidity with which it has 

 reached maturity and multiplied. 



I have since observed it in the canal between Warwick and 

 Birmingham, and it has also been found in the canals near 

 Wednesbury in Staffordshire. 



In all the cases hitherto cited, this shell has been found in 

 navigable waters, where its transport has doubtless been ef- 

 fected by means of timber. I only know of one instance to 

 the contrary, which is that of the Learn, at Leamington, where 



