Shell 6-8 mm long, with about five teleoconch whorls; spire tall, sutures barely impressed, body whorl strongly angulate at mid-whorl; labial edge denticulate within, columella with thin callus, two or three small ridges on angulation. Periostracum of living specimens forms axial ridges. Color tan, some specimens showing fine white mottling, some darker colored. Protoconch off-white to tan, 3-3.25 whorls, moderately developed velar sinus, last protoconch whorl wider than first teleoconch whorl.

The shell is similar to that of M variegata, but the protoconch is much larger, and the teleoconch whorls flatter, with a basal angulation. Pleistocene specimens described as A. tuberosa major reach a larger size (11 mm), compared to about 8 mm for living specimens.

Montague Island, Prince William Sound, Alaska, to San Pablo Point, Baja California. Intertidal in sand and gravel and in beach drift; more common subtidally, in gravel and under kelp.