One of the more common fossil bivalves
in NPL's Cretaceous
collections is the oyster Exogyra.
This fine specimen of Exogyra ferox from the Type
collection still has both valves (halves) of its shell.
Oysters are often preserved as fossils, probably because their
shells are thick, hard, and composed of stable
calcite, and because they lead a 'cemented' lifestyle.