The Collaborative International Dictionary
Quadrula pustulosa
Warty-back \Wart"y-back`\, n. An American fresh-water mussel ( Quadrula pustulosa). Its shell is used in making buttons.
Wikipedia
Quadrula pustulosa
Quadrula pustulosa, common name the Pimpleback, is a species of freshwater mussel, an aquatic bivalve mollusk in the family Unionidae, the river mussels.