Shell thin, medium to large sized (commonly exceeding to 10 cm in length), laterally compressed, almost circular in outline, gaping anteriorly and posteriorly. Both valves somewhat convex, the right (lower) valve only a little more inflated and larger than the left (upper) valve. Ears small, subequal in size and shape, with the right anterior ear faintly sinuated at anteroventral margin and devoid of ctenolium. Surface of left valve with 2 broad and very shallow depressed areas radiating from the umbo to anteroventral and radial lines. Interior of both valves with distinct radial ribs, usually in pairs, much narrower than the flat interspaces and becoming obsolete on umbonal area. Right valve with 42 to 54 internal radial ribs (46 to 54 in the typical subspecies A. japonicum, and 42 to 48 in the southern A. japonicum balloti). Colour: outside of left valve reddish brown, with variable shades along the concentric growth marks; outer colour pattern of subspecies balloti also with small, yellowish and dark brown spots along radial lines on umbonal area and often with a few irregular, larger dark spots towards periphery. Interior of left valve glossy to pale yellow externally and internally; subspecies balloti with an additional external brown scattering along some concentric growth marks.
