Shell thin and brittle, elongate oval, umbones anterior to midline; anterio-ventral margin deeply concave about a large, elliptical pedal gape, posterior margin regularly rounded, not gaping. Both valves strongly convex anteriorly. Sculpture of concentric ridges and radiating lines, produced as prominent tubercles where they intersect, most pronounced anteriorly; growth stages distinct in larger specimens. Umbonal reflection pronounced, with ten or more vertical septa linking it with umbone. Dorsal margin with a broad, oval protoplax, a small mesoplax and an asymmetric metaplax overlying the right side ( P. dactylus-drawing). Apophysis broadening towards its free edge, extending from beneath umbo, in each valve, halfway towards the ventral margin (P. dactylus-drawing 2). Adductor scars and pallial line clear, pallial sinus deep, U-shaped.
Up to 150 mm long.
Dull white or grey, periostracum yellowish, often discoloured. Inner surfaces white.
The animal is whitish. The siphons are large, united nearly to the fringed orifices and covered with a brown or speckled sheat. The foot is flattened, the rasp-like forepart assists in boring. The hinge-plate is continued outside, and folds over the upper margin of the shell, where it is protected by a portion of the mantle. To protect the mantle, the accessory plates cover it at this part. The animal is somewhat club-shaped.
Bores into a wide range of substrata including wood, peat, compacted sand, and various soft rocks. Lower shore and shallow sublittoral.
Ranging to the Mediterranean and north-west Africa (Distr. P. dactylus).