|Arcidae, the ark clams, have shells that vary in shape, from elongate to ovate, usually very inflated. A common characteristic is their radiating ribs. The shells are white or off-white in color but are usually covered by a brownish, hairy outer skin (periostracum). The hinge line is usually long and straight with many small "teeth" or dentations. They are one of the most abundant family of shells found on Texas beaches, probably because some live buried in the shallow, intertidal zones. Others, though, use byssal threads to attach themselves to hard substrates, such as rocks and coral. There are 15 species found in Texas.