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