The best known of the group of bivalves that attach themselves by cementing their shells to the substrate (which can be a hard surface of any type, rock or shell). They are in the Order Pteroida, Suborder Ostreina. Oysters are one of the most successful bivalves, and are abundant in the Mesozoic and Cenozoic eras. Many examples from the Cretaceous of central Texas are preserved in life position. ( Bivalves)