These are a species of ancient oysters know as Rastellum Carinatum. The wide angled ribs have led to this guy being called "Denture Clam." The zig-zag pattern that joins the two halves of the shell stopped coarse dirt and debris from entering the shell and damaging its soft body. Similar to modern day oysters, they lived in shallow coastal waters and fed off of food particles that it filtered from the water. These oysters date from the Cretaceous Age, approximately 70 million years ago.