Shell small for genus, thin, ovately rounded in outline; lunule proportionally large, heart-shaped, bounded by incised line; inner margin of commissure crenulate; umbones recurved, strongly prosogyrate; shell exterior sculptured with numerous strong, rough, raised concentric growth lines and large, widely separated concentric lamellae; prominent large, smooth, and shiny semicircular patch present in area ranging from edge of commissure to center of valve; shiny semicircular patch bounded anteriorly and posteriorly by strong raised lamellae; shell color pale yellow-cream, with pale purple amorphous patches and bands present on escutcheon and near umbones; pallial sinus deeply reflexed and sharply pointed; interior of shell pale yellow-cream, often marked with band of pale purple along posterior edge of commissure; both valves with 3 cardinal teeth, with central tooth bifurcated by deep groove. Holotype: Width 44.28 mm, height 39.51 mm, FMNH 328438; additional specimens, width 40 mm, same locality as holotype, in the research collection of the author; width 42 mm, in the Jane Hart collection.

This previously overlooked Mercenaria species is confined to the area of the Palm Beach Provinciatone (from Fort Pierce south to Lake Worth), where it is an inhabitant of muddy coastal lagoons and inland tidal creeks near red mangrove forests. Throughout its distribution, Mercenaria hartae is sympatric with the wide-ranging Mercenaria campechiensis (Gmelin, 1791) but can be easily separated by several consistent differences in shell morphology: (1) M. hartae is a much smaller and flatter species with a thinner shell; (2) M. campechiensis is heavily sculptured with fine concentric ribs and lamellae over its entire shell surface, while M. hartae has a very prominent and distinctive shiny and unsculptured semicircular patch that extends from the commissure and edge of the shell to the center of the valve; (3) the shell of M. hartae is proportionally much wider and more rounded than that of M. campechiensis and is more dorsoventrally compressed; and (4) the concentric lamellae of M. hartae are proportionally larger, more prominent, and more widely-separated than the closely-packed lamellae seen on M. campechiensis.
This new Georgian Subprovince venerid species is the ecological and morphological analogue of Mercenaria texanum (Dall, 1902) from the Texan Subprovince. Both endemic species are restricted to intertidal environments in coastal bays and inlets and both are smaller and more rounded than the sympatric M. campechiensis. Similarly, both M. texanum and M. hartae have smooth areas of reduced concentric sculpture. Of the known Mercenaria species of the Carolinian Province (discussed in Chapter 2), only M. hartae has the distinctive, highly polished semicircular patch bordering the shell margin.

Type Locality: On intertidal sandy mud flats near red mangrove forests along Pine Point, Singer Island, Lake Worth Lagoon, Palm Beach County, Florida.