The pearl oysters are pinctada and belong to the other side of the family, more closely related to the scallop and pen shells. The difference is that like mussels the pinctada attach themselves to stable objects like a rope or root in the water by a filament, where the ostrea has an all important foot which allows scientists to conjecture that early in evolution it might have moved about by itself. The pinctada would always have been sessile. They are known as winged oysters from the Greek pteria. Certainly pearl divers ate them, but they are not considered worth cultivating as food.
The fabled but now rare English and European native oyster is the Ostrea edulis. They are flatter, rounder and usually more elegant, and smoothly shelled than the more commonly found crassostria which are either the native American Crassostrea Virginica or they are Pacifics from where they came or increasingly shorthanded down to rock oyster. There is another ostrea, native to the west coast of north America, the smaller lurida. They are usually called Olympia after the City of Olympia on the Puget Sound, Washington, near where they are mostly cultivated, although they have been found from south east Alaska round to Baja California. These may only be the size of a 50p coin but can still take nearly five years to mature and are held in fond reverence in America culinary history.
Geologically speaking the ostrea is the oldest of oysters dating back to the late Palaeozoic times and abundant in Jurassic and Creteceous periods. To put numbers on that, the Palaeozoic era was 542 to 251 million years ago. The oyster is an extraordinary survivor.