208 BRITISH MOLLUSKS. 



posterior margin of each valve, hinge toothless, cartilage ex- 

 ternal, marginal. 



The tranquil and gently running waters of northern and eastern 

 Europe, between the Baltic and Caspian Seas, are inhabited by a 

 peculiar and most prolific mussel, which has been transported 

 among timber and on ships' bottoms to the western parts, and has 

 become naturalized in all corners of the British Isles. It differs 

 from the sea mussel (Mytilus) and from all the rest of the unimus- 

 culose Acephala, in the important feature of having the mantle 

 closed throughout, excepting at the three orifices necessary for the 

 passage of the foot, for excretion, and for respiration. In the 

 Mytili the mantle is open, the free lobes on either side correspond- 

 ing with the valves of the shell. In Dreissena, the shell is secreted 

 by a closed integument, on which the mantle or calcifying portion 

 of it is indicated by a linear fringe of papillae. 



Dreissena, named by M. Van Beneden, a Belgian naturalist, 

 after a M. Dreissen, was first noticed more than a century ago in 

 the river Yolga, by the eminent G-erman traveller and naturalist, 

 Pallas, during a voyage in Russia. It was not detected in Britain 

 until 1824, when Mr. J. De Carle Sowerby acidentally observed a 

 gentleman fishing for perch with it, as bait, in the Commercial 

 Docks. The mollusk had been imported from eastern or northern 

 Europe, probably across the Baltic, either among timber or attached 

 to the ship's bottom, and was abundantly multiplying. From the 

 habit of Dreissena living in brackish as well as in fresh water, it is 

 believed to be capable of enduring salt water for a time, as well as 

 living for some time out of water, and thus it traverses the sea in 

 ships or adhering to ships, and it has become naturalized in most of 

 our docks, lakes, canals, reservoirs, and even in our water-pipes. 



It will be seen on reference to our vignette, that the anterior or 

 lower orifice is merely a simple opening for the passage of a small 

 foot, which is comparatively short and conoid when contracted, 

 tongue-shaped when extended. From a groove in this foot, a copious 

 fibrous byssus is spun for the purpose of adhesion to foreign bodies. 

 Of the two hinder or upper orifices, the one for ejecting the water is 

 developed into a simple conoidal siphon, the other for taking it in 

 is more complicated. The portion of the mantle in which it is 

 situated, protrudes in a subtubular manner, and is armed with lon- 

 gitudinal rows of irregular scale-like papillae. At the orifice it is 

 contractedly pouted, and rimmed with papilla?, those on the inner 

 edge being elongated in a more or less crowded manner towards 



