Ark Shells in the Arcidae Family are strongly ribbed clam shells. The two sides of the clam are symmetrical and look like an ark or treasure chest, from which they derive their common name. Ark Shells in the Arcidae Family differ from Ark Shells in the Noetidae Family in the placement of ligaments and the lack of striations on the ligaments. The hinge line on Ark Shells is arched and there are numerous teeth along the hinge line of both valves that align, like a zipper, when the shell is closed. Many ark shells are covered with a heavy periostracum (outer membrane), usually dark brown in color. Ark Shells can be found crawling along sand or mud, or attached to stones or rocks by byssus (thread-like anchor lines). The Ark Shells have a global distribution and are found from shallow to very deep waters. There are two hundred species in the Arcidae Family of which twenty-nine are found along the coastlines of the Baja Peninsula.