This fossil is an oyster shell, called a valve, from the Albian age which occurred in about the middle of the Cretaceous period.
Oysters are a type of bivalve, a form of creatures that have inhabited Earth since the Mid-Cambrian, about 300 million years before dinosaurs came along.
Bivalve shells are made from layers of crystals of the minerals calcite or aragonite, which are the two different mineral forms of calcium carbonate (CaCO3). Different crystal layer arrangements help them adapt to a variety of environments. The most important functions of fossil bivalve shells were to protect against predators and prevent dehydration in intertidal environments.
Some oysters are hermaphroditic, both male and female, and some can switch between sexes depending on the water temperature. A single female oyster can produce up to 100 million eggs annually.
110 Million Year Old Oyster Valve