The seller says that at 52 inches across, this is the world's largest fossil oyster [right]. It's from the Cretaceous Niobrara Fm. in Gove Co., Kansas. The flyer says these oysters were unusually thin, which allowed them to 'float' along the sediment surface of the ancient sea.
The Fossil Hall at
the Inn & Suites hotel two blocks from the AZGS building is one
of the more spectacular venues of big and exotic fossils. I ran down during lunch and, as usual, was blown over by what's for sale. The dinosaur skull on the lower right has a 'sold' sign on it, as did a number of other showy pieces including a beautiful mammoth tusk.