20 DREISSENID&. 



inside whitish ; hinge with small teeth, or toothless ; each valve 

 has a triangular depression below the beak for the reception of 

 the anterior muscle. 



The Dreissenida bear a strong resemblance to the 

 marine Mytilidce (mussels), but differ from them in 

 the mantle being almost closed and in having a trans- 

 verse plate on the hinge inside the shell. 



GENUS DREISSE'NA* VAN BE NED EN. 



The Dreissencz have the power of living for a con- 

 siderable time out of water ; they are gregarious, and 

 attach themselves to surrounding objects by a byssus, 

 as the marine mussels do. A single species only, 

 belonging to this genus, occurs in Great Britain, or 

 indeed in Europe ; it was discovered in 1754 by 

 Pallas, a Russian naturalist, in the River Wolga, as 

 well as in the Black Sea. In this country it was first 

 noticed in 1824, in the neighbourhood of London, and 

 was supposed to have been imported in timber ships 

 from the Baltic. Gwyn Jeffreys, however, inclines to 

 the belief that it is indigenous. 



DREISSENA POLYMOR'PHA^ PALLAS. PL. TIL 



Body usually blackish : foot oblong, cylindrical, narrow, 

 slightly transparent, grey faintly tinged with rose-colour ; mantle 

 bordered with grey in front, marked behind with dark brown 

 stripes ; gills grey ; byssus strong. 



Shell triangular, sharply keeled in the centre of both valves, 

 somewhat compressed below, gradually expanding obliquely 

 downwards, somewhat solid, of a dullish aspect, yellowish-brown, 

 usually marked above with wavy lines of a purplish or brownish 

 hue, strongly wrinkled in the line of growth, faintly and irregu- 

 larly puckered ; epidermis silky ; beaks small, terminal, incurved ; 



* Named after M. Dreissens. t Many-shaped. 



