The giant pacific cockle grows to about 80 mm. It has about 40 stong radial ribs each with a row of sharp triangular spines. Color ranges from dull gray to yellow.

Usually uncommon to rare in firm sand or mud shallowly buried, may pop up from the sand if disturbed. This specimen was found resting on mud stone in a sampling quadrat at SCAR.

Intertidal to 20 m; Monterey Bay, California to Southern Baja California, Mexico.