Dreissena, a genus of mussels, differing from. the common Mgtilus edulis by the absence of the pearly internal layer. The best-known species, D. polyinorpha, is a brackish-water inhabitant and a native of the rivers of the Aralo-Caspian area. It was first observed in England in the Commercial Docks (Surrey) about 1824, but it has since spread into most of the rivers and canals. It appears to have been introduced attached to timber.