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Bivalves belong in the class Bivalvia in the phylum Mollusca. "Valve" refers to the shell, thus bivalves have two shells, but so do some other mollusks. In bivalves, the two shells are right-handed and left-handed, mirrors of each other, and each shell is asymmetrical. (The other two-shelled mollusks, the brachiopods, have two unmatching valves, each one symmetrical.) Bivalves are among the oldest hard fossils, showing up in Early Cambrian times more than 500 million years ago. It is believed that a permanent change in ocean or atmospheric chemistry made it possible for organisms to secrete hard shells of calcium carbonate.