Both valves of the shell are convex. The outline is rounded with conspicuous ribs and projections, ears, on each valve. It can grow up to 9 cm in diameter. The shell is variable in colour but often light-pink to brown, orange or yellow and often with bands, zigzags, rays and spots of darker or lighter shades. The right valve is often paler and flatter than the left. The ears of the shell have fine ribs and concentric corrugations. The sculpture of the shell consists of approximately 20 bold, radiating ridges and the margins of the shell are strongly crenulate. When viewing the inside of the right valve (the more convex shell) the left (or anterior) ear is larger than the right. The left forms a notch below the ear and possesses small teeth. It is from this notch the young scallop release fine threads (the byssal thread) to attach to hard substrata. Spines are absent. The shell is often overgrown with encrusting sponge.Young scallops remain attached by the byssus, later becoming unattached and are then able to swim freely. The queen scallop differs from the king scallop Pecten maximus. In the king scallop, the lower valve on which it lies, is deeply convex and white, while the upper valve, generally red or brown (often marbled) is almost flat. In the queen scallop, both valves are convex, the upper being slightly more-so than the right and colour is variable. The number of ribs varies between species with the queen scallop possessing 19 to 22 ribs and the king scallop 15 to 17 ribs. The queen scallop is fished commercially at a number of localities and particularly around the Isle of Man. Often the valves are heavily encrusted with various organisms particularly sponges. This relationship has been described as protective-mutualism. The sponge is thought to protect the scallop from predation by starfishes while the sponges are protected from predation by the sea slug, Archidoris pseudoargus. Scallops detach from their byssal thread at some point in their life, usually upon attaining 1.5-2.0 cm but retains the ability to secrete a byssus until at least 6.5 cm long.