This species may grow to 50 mm (about 2 inches.) Its shell is oval, with a sculpture of fine commarginal (“concentric”), regularly spaced ridges. The shell color is brown. It differs from the Southern Marsh Clam, Polymesoda floridana, by having a more rounded shell with truncated posterior end (left end of leftmost views on photo). This is a truly brackish water species, inhabiting the upper reaches of local estuaries (the shell illustrated was collected in the Caloosahatchee River estuary.) Photos by Patricia A. Starkey.