Pinnids, ostreids, and pteriods are others that have at least part of
the shell flexible in life.  Because the shell margins are somewhat
flexible, the valves don't need to align precisely to ensure closure,
and they have weak hinge teeth.  In oysters, it's the thin edge that
is flexible; it's often lost after death, making the valves look
unequal in outline.

--
Dr. David Campbell
425 Scientific Collections
University of Alabama
"I think of my happy condition, surrounded by acres of clams"

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