Ostreida

thumb|Mixed seafood in Dubai; oysters are at the edge of the tray Oyster is the common name for a number of different families of salt-water bivalve molluscs that live in marine or brackish habitats. In some species, the valves are highly calcified, and many are somewhat irregular in shape. Many, but not all oysters, are in the superfamily Ostreoidea.

The order Ostreida includes the true oysters. One superfamily (Ostreoidea) and two extant families are recognised within it. The two families are Ostreidae, the true oysters, and Gryphaeidae, the foam oysters.

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.

genus of molluscs