A Mussel

1. Any of several groups of bivalve shellfish with elongated, asymmetrical shells

• fresh water mussels, usually edible, of the order Unionida in subclass Palaeoheterodonta.
• salt water mussels, usually edible, of the order Mytilida in subclass Pteriomorphia.
• Certain other bivalves of somewhat similar appearance, such as the zebra mussel and quagga mussel of the family Dreissenidae in subclass Heterodonta.