What is "forkshell"

The forkshell or Lewis pearly mussel, scientific name Epioblasma lewisii, was a species of freshwater mussel, an aquatic bivalve mollusk in the family Unionidae, the river mussels.

This species is endemic to the drainages of the Cumberland River and the Tennessee River in the United States. Its natural habitat was rivers. It became extinct due to habitat loss.