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 .)
![](https://static.wixstatic.com/media/f867c2_7875daec093d4186b917162e77c358e2~mv2.jpg/v1/fill/w_141,h_92,al_c,q_80,usm_0.66_1.00_0.01,blur_2,enc_auto/f867c2_7875daec093d4186b917162e77c358e2~mv2.jpg)