Thick-shelled edible clam of the U.S. The northern quahog (Mercenaria mercenaria), also known as the cherrystone, littleneck, or hard-shell clam, is 3–5 in. (8–13 cm) long. The dingy white shell is thick and rounded and has prominent concentric lines. It is found in the intertidal zone from the Gulf of St. Lawrence to the Gulf of Mexico. It is the most important food clam of the Atlantic coast. The southern quahog (M. campechiensis), found in the intertidal zone from Chesapeake Bay to the West Indies, is about 3–6 in. (8–15 cm) long and has a heavy, white, plump shell.