Broadly trigonal, compressed, thin-walled, brown, with coarse radial ridges adorned with inrolled spines on about half of the valve, central muscle scar well within internal nacreous area, mantle beige to pale yellow. North Carolina to Florida, West Indies, Gulf of Mexico, Caribbean Central America, South America (to Argentina). Length 180 mm (to 243 mm). Compare Atrina rigida, which has an orange mantle and a muscle scar at or above the edge of the inner nacreous layer.