Dreissena

Dreissena, a genus of bivalves, in the mussel family (Mytilidæ), peculiar in having the mantle halves almost completely united. One species (D. polymorpha) deserves notice, since, from its home in the Black Sea and Caspian, it has travelled westward on ships or logs, and has now established itself in many European (including British) estuaries and canals.