I immediately recognized this next shell when I first unwrapped it from the parcel it came in. It belongs to the genus Exogyra and likely is the species E. costata. It comes from the Corsicana formation (Cretaceous, Maastrichtian stage) near San Antonio, Texas. It looks very similar to E. costata shells found in the Navesink thru Mt. Laurel formations ( also Cretaceous, Maastrichtian stage) of New Jersey and Delaware like this one.
Left valve top
Right side profile
Left side profile
Interior of left valve. It still has a slight mother of pearl sheen to the interior!
I received this in a batch of fossils that I won in an auction on the Fossil Forum site. Thanks, Travis, for the cool fossil!