48 DRETSSENID/E. 



much incurved : ligament long and narrow, fitting into a groove 

 of the hinge in each valve : upper margin angular : anterior 

 side nearly straight : posterior side curved : lower margin in- 

 curved, and forming in the middle a large slit for the passage 

 of the foot and byssus : inside slightly nacreous : hinge strong, 

 toothless, but furnished inside each valve with a triangular 

 and concave plate which is placed under the beak : muscular 

 and j9aZZmZ scars indistinct. L. 1*4. B. 0-6. 



Habitat : Slow rivers, canals, and lakes in the eastern, 

 home, midland, and northern counties of England, as 

 well as in a canal near Worcester (Reece), Bath (Hutton), 

 and at Edinburgh. In the North of France, Belgium, 

 and Germany it is also common and widely diffused. 

 In one respect this species may be said to be truly metro- 

 politan; for it has been found in the most frequented 

 streets of London, after they have been flushed with 

 water from the New Biver, where it abounds. Mr. 

 Norman informs me that he saw immense numbers of 

 the Dreissena in a living state, lining some of the iron 

 water-pipes which had been taken up in Oxford Street, 

 and that the colouring of the shells was as vivid as if the 

 animal had lived in the light of day. 



It is difficult to believe, in the absence of some proof 

 to the contrary, that this species is not indigenous to 

 the whole of the North of Europe, as well as to Bussia. 

 The circumstance of its not having been noticed in this 

 country before 1824, and then only in a metropolitan 

 locality, does not preclude the possibility of its having 

 previously existed in some other part of Great Britain ; 

 and its not having been previously recorded as British 

 rather proves a want of observation or opportunity than 

 its non-existence. Helix Cartusiana, H. obvoluta, and 

 Clausilia Rolphii, all of which are conspicuous land shells, 

 were not known to the observant Montagu, although 

 they are not uncommon in some parts of this country 



