Flat Pigtoe

Pleurobema marshalli, the flat pigtoe or Marshall's mussel, was a species of freshwater mussel in the family Unionidae, the river mussels. It was native to Alabama and Mississippi, but it has not been seen since 1980. Though it is still listed as critically endangered on the IUCN Red List and as an endangered species on the US Endangered Species List, it is likely extinct.

This mussel was last seen in a stretch of the Tombigbee River before the habitat was destroyed by the installation of the Tennessee-Tombigbee Waterway. No living or freshly dead specimens have been seen since.