It is oval shaped with 8 separate moderately elevated, overlapping ridged valves on its dorsal surface. It resides along the coast of North America.
The distribution of Mopalia ciliata is somewhat controversial, however it is agreed that it resides on the southern coast of North America as far south as Baja California. The presence of this species north of California is currently debated whether it be a similar species Mopalia Kennerleyi. Mopalia ciliata are found on the underside of rocks near cracks or crevices or other hard stubstrata in intertidal zones. This species is an omnivore. They feed on algae, bryozoans, hydroids and other low-growing organisms.