Ostreidae

Ostreidae are the true oysters. The family includes most species eaten under the name "oyster". Pearl oysters are not true oysters because they belong to a different order, the Pterioida.
Like scallops, true oysters have a central adductor muscle. This causes a scar in the middle of the shell. It is the point of attachment. The shell is often irregular because oysters attach to substrates.