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Bivalves, classified among the mollusks, are common fossils in all rocks of Phanerozoic age. (more below)
The classic shellfish dates from Cambrian times
Photo (c) 2005 Andrew Alden, licensed to About.com (fair use policy)
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. This fossil clam is young, from the Pliocene or Pleistocene rocks of central California. Still, it looks just like its oldest ancestors.

For lots more detail on the bivalves, see this lab exercise from SUNY Cortland.

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