Shell elongate, very irregular; anterior end deeper than posterior, umbones prominent and widely spaced, each with a sharp, raised ridge running from the beak to posterior margin. Cardinal area broadly triangular, with distinct chevron ridges, partly obscured in older specimens. Sculpture of fine radiating ridges; older shells worn to a rough, chalky white surface, coarse and rugose between umbones. Hinge line with 40-50 teeth, intersecting the rounded anterior margin at right angles, forming a sharp step with concave posterior margin. A small ridge extends from posterior end of hinge line to umbones. Ventral margin irregular and variable, usually convex, occasionally concave, finely crenulate, with a distinct byssal gape ventral to umbones. Adductor scars prominent, often in relief (A. tetragona-drawing).
Up to 50 mm long.
Off-white or yellow, periostracum brown. Inner surfaces dull brown in smaller shells, off-white in older shells, with bluish, pale brown, or green blotches.
Whitish mantle with a number of closely grouped ocelli towards the hinder margin. These dark spots are not true eyes but are so sensitive to changes in the intesity of light that they serve the same purpose. The conical white foot has a deep central groove, through which a dark green byssus is spun. (A. tetragona-animal).
Sessile, attached by a thick, green byssus; in crevices, among rocks and shell debris, often encrusted with other organisms, or abraded by the substratum. From the lower shore to about 100 m depth.
Distributed from Norway south to the Mediterranean and west Africa (Distr. A. tetragona).