Family Glycymerididae

Shell size to 30 mm; shell circular, compressed. Sculpture of 20-35 ribs flat in cross-section. Ribs wider toward middle of shell, interspaces much narrower than ribs. Hinge teeth about 10 on each side. Color variable, white to grayish-white with irregular brown streaks of variable intensities. Bittersweet clams (family Glycymerididae) have numerous, simple eyes that resemble those in insects. They can be seen as tiny dark dots along the mantle edges of the clam in the supplementary photo, which was taken by Amy Tripp in March, 2016 on Kice Island, Collier County, Florida.

Shell size to 30 mm; shell elliptical, compressed. Sculpture of 30-40 flat, low ribs. Hinge teeth about 10 on each side. Color ochre- to greenish-brown. Differs from Glycymeris pectinata by more elliptical shell, larger number of ribs, and uniformly brownish color.

Shell size to 100 mm; shell large, almost circular, compressed. Umbones at midpoint of hinge. Hinge teeth reduced under umbones. Sculpture of numerous low ribs. Margins of valves crenulated. Color white-gray, drab-gray, or tan, usually with rusty-brown markings. The periostracum is brown, velvety. The photo of the live bittersweet was taken in a research cruise in the NE Gulf of Mexico in 2021.