Shell is flat, inequivalve, right (lower) valve is thin, sometimes fragile,
left (upper) valve is solid. Inequilateral, beaks central but set back from the margin, early growths clear. Outline is irregular though tending to be circular. often overgrown. Byssal aperture is pear-shaped. Ligament pit is half-moon-shaped. Upper valve with 30 or more radiating ribs, lower valve with fine concentric lines. Interior of upper valve with radiating furrows on the adductor and byssal muscle scars, which are quite separate and distinct from each other, lower valve with one adductor muscle scar (P. patelliformis-drawing).
Up to 38.1 mm in diameter.
White, sometimes with red-brown markings. Interior of upper valve is glossy white, lower valve translucent. Muscle scars and adjacent area in the upper valve may be stained yellowish green.
On hard ground and dead shells, to considerable depth.
Occurs from the Norwegian Sea south to the Mediterranean (Distr. P. patelliformis).