The Ostreidae, the true oysters, include most species of molluscs commonly consumed as oysters. Pearl oysters are not true oysters, and belong to the order Pteriida.
Like scallops, true oysters have a central adductor muscle, which means the shell has a characteristic central scar marking its point of attachment. The shell tends to be irregular as a result of attaching to a substrate.
Pinctada is a genus of saltwater oysters, marine bivalve mollusks in the family Pteriidae. These pearl oysters have a strong inner shell layer composed of nacre, also known as "mother of pearl".
Pearl oysters are not closely related to either the edible oysters of family Ostreidae or the freshwater pearl mussels of the families Unionidae and Margaritiferidae.
Ostreoidea is a taxonomic superfamily of bivalve marine mollusc, sometimes simply identified as oysters, containing two families. The ostreoids are characterized in part by the presence of a well developed axial rod. Anal flaps are known to exist within the family Ostreidae but not within the more-primitive Gryphaeidae. The scar from the adductor muscle is simple, with a single, central scar. In the majority, the right valve is less convex than the left.