[mus.sel] n [ME muscle, fr. OE muscelle, fr. (assumed) VL muscula, fr. L musculus muscle, mussel] (bef. 12c) 1: a marine bivalve mollusk (esp. genus Mytilus) usu. having a dark elongated shell
2: a freshwater bivalve mollusk (as of Unio, Anodonta, or related genera) that is esp. abundant in rivers of the central U.S. and has a shell with a lustrous nacreous lining