Diagnostic characters: Shell mostly solid, equivalve or subequivalve, obliquely rounded, or ovate to subtrigonal in outline and usually not gaping; inequilateral, with generally prominent,
prosogyrate umbones, at or in front of the midline of shell. Lunule and/or escutcheon usually present. Sculpture only concentric, or with a radial component. Periostracum most of the time inconspicuous. Ligament external, behind the umbones, often inserted in a deep groove. Hinge with 3 usually radially disposed cardinal teeth in each valve (1 or more of which may be grooved or bifid), anterior lateral teeth sometimes present. Interior of shell porcelaneous. Two more or less equal adductor muscle scars, the posterior sometimes slightly larger. Pallial sinus usually present. Internal margins smooth to denticulate. Gills of eulamellibranchiate type, with folded branchial sheets; outer demibranch smaller than the inner, expanded and almost flat above the axis. Foot large and rather short, hatchet-shaped, rarely byssate in the adult. Mantle broadly open ventrally. Siphons short to long, naked, fused or separate, with simple tentacles on tips and inside the inhalent opening to strain out large particles.