river mussel

Short definition from Wiktionary

Noun

1. 1
   Certain Any bivalve mollusks of family Unionidae, found in flowing fresh water: especially of genera Unio and Anodonta, both found in rivers of the central U.S. with a shell lined with mother-of-pearl.
2. 2
   Certain Any bivalve mollusks of family Unionidae, found in flowing fresh water: especially of species Alathyria jacksoni.