Atrina sp. - fragmentary pen shell from the latest Holocene of Florida, USA.
<p>Bivalves are bilaterally symmetrical molluscs having two calcareous, asymmetrical shells (valves) - they include the clams, oysters, and scallops.  In most bivalves, the two shells are mirror images of each other (the major exception is the oysters).  They occur in marine, estuarine, and freshwater environments.  Bivalves are also known as pelecypods and lamellibranchiates.
</p><p>Bivalves are sessile, benthic organisms - they occur on or below substrates.  Most of them are filter-feeders, using siphons to bring in water, filter the water for tiny particles of food, then expel the used water.  The majority of bivalves are infaunal - they burrow into unlithified sediments.  In hard substrate environments, some forms make borings, in which the bivalve lives.  Some groups are hard substrate encrusters, using a mineral cement to attach to rocks, shells, or wood.
</p><p>The fossil record of bivalves is Cambrian to Recent.  They are especially common in the post-Paleozoic fossil record.
</p><p>Seen here is a fragment from a pen shell, Atrina sp. - this is the interior surface.  Atrina bivalves are known in the fossil record - their reported age range is Triassic to Holocene.  Atrina shells are thin, moderately fragile, and relatively organic-rich (Atrina shells can be partially dissolved by the dilute bleach that is used to clean these and eliminate foul odors).  The interior surface of individual Atrina shells has attractive, rainbow-iridescent nacreous aragonite (CaCO3 - calcium carbonate).
</p><p>Classification: Animalia, Mollusca, Bivalvia, Pteriomorphia, Pterioida, Pinnidae
</p>
Locality: Lighthouse Point beach, southern shore of the eastern tip of Sanibel Island, Gulf of Mexico coast of southwestern Florida, USA
James St. John
CC BY 2.0
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