You can recognize Derbyia by its fine radiating lines called costae that extend outward from the beak (top of shell in photo). The beak is where the two shells or valves meet in a long, straight hinge. Much of the hinge is not visible in this photo. See if you can find a Derbyia with visible hinge. Some specimens are very large, up to 4 inches across. Both valves of the shell are quite flat.