Polymesoda caroliniana Bosc, 1801 may grow to 50 mm (about 2 inches.) Its shell is oval, with a sculpture of fine commarginal (“concentric”) 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 near downtown Fort Myers .)
The Carolina Marsh Clam, Polymesoda caroliniana. Photos by Patricia Starkey.
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