OK, Conchlanders, here's a simple question that may be hard to answer. What
are the thickest shells? The ones with the thickest walls of calcium
carbonate, that is--not the thickest body.
 
I vote for the Cretaceous gryphaeid oysters, Exogyra ponderosa and
Pycnodonte mutabilis. These Gulf Coast shells can be about 4 cm thick or
more; I don't know if modern oysters can match that. Oysters can't move to
escape predators, so the extra shell thickness may be very necessary.
 
Andrew K. Rindsberg
Geological Survey of Alabama