The heavy shell of the southern quahog, Mercenaria campechiensis, is ovate trigonal and inflated with valves of equal size (Andrews 1994). Several concentric growth lines are present on the gray to whitish exterior. Three cardinal teeth (the middle one split) on each valve help to hold the shell together at the hinge. In empty shells, the interior color is often porcelaneous white with purple marks occurring rarely. Two muscle scars are present on the interior surface, both attached to the pallial line.