 Freshwater zebra mussels and their relatives in the family Dreissenidae are not related to previously mentioned groups, even though they resemble many "Mytilus" species in shape, and live attached to rocks and other hard surfaces in a similar manner, using a byssus.They are classified with the Heterodonta, the taxonomic group which includes most of the bivalves commonly referred to as "clams".The mussel's external shell is composed of two hinged halves or "valves".