Fusconaia subrotunda, the longsolid,[2][3] long solid mussel or long solid naiad, is a species of freshwater mussel, an aquatic bivalve mollusk in the family Unionidae.

This mussel, sometimes called a pigtoe, is found in river gravel.

Young mussels are light brown, but darken to nearly black with age.

This species is found in Canada and the United States.

This Unionidae-related article is a stub.