OSTREA EDULIS
The Belon is the most well known of these oysters. They are round, symmetrical, expensive, and always flavorful. They should never be cooked. They will not be available in markets during the summer (the non-R months) because this is the time when the oysters spawn, causing the texture to become gritty and the flavor to diminish.

Also called: European oyster, flat oyster
In England: Dorset, Whitstable
In America: they have been successfully cultivated, especially in Blue Hill (Maine) and in the Pacific Northwest
In France: Belon -- the classic example of Ostrea edulis and, for some, the epitome of oyster eating. The Belon is native to Brittany.

Marennes -- the famous green-tinged oysters are raised in claires, with a high concentration of the cholorophyll-containing diatom Navicula ostrearia (a tiny algae). They are grouped as fines de claires, which, according to the rather complicated French system of classification, means they are designated as the most superior in quality of oysters.

OSTREA LURIDA
It seems that the only known type of Ostrea Lurida is the Olympia, an oyster that is native to the Pacific Northwest of the United States, and that is mostly associated with the waters of Puget Sound in Washington State. It is a very tiny oyster -- on average about the size of a quarter, certainly never larger than two inches in diameter. It takes up to five years to mature, a relatively long time. The flavor is renowned for being good and strong, with a pleasant aftertaste. Again, this oyster is too fine to be used for cooking. As with the European oyster, the Olympia spawns over the summer and changes in texture, making it less desirable during those months.

CRASSOSTREA VIRGINICA
This is the species that most American oyster lovers call their own. They thrive all the way up to Prince Edward Island and down into the Gulf of Mexico, so the variation among oysters of the different regions is quite great. In general, northern oysters are considered to be firmer, somewhat better for eating raw than the southern varieties. This difference is attributed to the colder climate of the north. Correspondingly, though they are safe to eat year round, some people do consider the softer texture and blander flavor of the oysters (even northern ones) in the summer to be less appetizing than during the "R-months."

Also called: Atlantic oyster, Eastern oyster
Regional names (just a few of many): Bluepoint (Long Island), Box Oyster (Long Island), Chesapeake Bay, Chincoteague (Virginia), Cotuit (Nantucket), Kent Island (Maryland), Malpeque (PE Island, Canada), Patuxent (Maryland)

Apalachicola (Florida) -- Plump and sweet, with a hint of copper flavor, these oysters have a greenish, deep shell. This may have been the oyster that was used for Oysters Rockefeller, a dish created at the New Orleans restaurant, Antoine's, during the Gilded Age. It was named after John D. Rockefeller, Sr., because they were both so "rich."

Breton Sound (Louisiana) -- These gulf oysters grow wild in the reefs of Louisiana. They vary in saltiness according to the season, becoming sweetest in the spring when the marshes are flooded with fresh water.

Wellfleet (Massachusetts) -- The salt marshes of Wellfleet in Cape Cod, fed as they are by springs, are the perfect environment for raising oysters. Although the oysters native to the region were depleted by the settlers there in the late 18th century, the area was quickly and successfully restocked with oysters from nearby areas and has remained a strong producer ever since.

CRASSOSTREA GIGAS
This oyster is, with a few exceptions, regarded as inferior for eating raw. It can grow to be 12 inches long, and is therefore often simply too large and tough to be good on the half shell. It adapts well to different environments and is widely cultivated outside of Japan, mostly on the West coast of the US.

Also called: Pacific Oyster, Japanese Oyster

Popular varieties for serving raw include: Mad River and Tomales Bay (California) and Totten (Puget Sound, Washington)

Kumamoto oyster -- This variation of the Japanese oyster is sometimes given its own species name, Crassostrea kumamoto. It was originally cultured on the island of Kyushu in Japan and is now raised in America from the Gulf of Mexico to British Columbia. It has a distinctive, frilly black shell, and the meat is praised as being delicate, even buttery.

 article is taken from http://www.cuisinenet.com