Any fresh-water mussel that produces a fresh-water pearl or, more often, mother-of-pearl, and especially the genus Unio that is found in the Mississippi Valley of the United States (as well as thewarty-back mussel) and the rivers of Scotland, north Wales, and Ireland, and parts of Germany, Austria, and Norway. Pearls are sometimes produced, but they are inferior tothose of the pearl oyster. The mother-of-pearl is used in great quantities to make buttons.