There are four main varieties to look out for:
La Plate ('the flat'): for centuries this was the only oyster to be had on the coast of France, but a disease caused their disappearance in the 1970s. Quite rare today, you find them sold as belon in Brittany and gravette in the Arcachon Bassin and Bouzigues. The wild variety is called pied-de-cheval ('horse hoof'). L'huître plate can be recognised by its round shell and grey-white flesh. It has a full-bodied flavour, very salty and should be prepared with the greatest simplicity.
'Take a health cure - eat oysters' is what is says on the bag.La Creuse ('the hollow'): This is by far the most common and the most affordable. They vary subtley in flavour depending on their geographical origin - salty and nutty from Normandy, iodiney and recalling the shallow estuaries in Brittany, delicate and invigorating from the Atlantic coast. Allow 6-8 oysters per person if you are eating them raw, and 5 if they are served warm. They are numbered from 0 (the biggest and fattest) to 6 (the smallest). On a platter, numbers 3-4 are ideal (below).
La Pousse en Claire ('the saltmarsh grown'): A little gem from the Charentes, protected by a Label Rouge certification, this is the most desirable variety, unlike any other. Raised from 4 to 6 months in shallow water, only 2 to 5 oysters per square metre, they acquire an almost crunchy firmness. Generously filling out their semi round shell they have a mild sweet taste. If you find them, eat them as they are, as they need nothing to enhance them.
La Fine ('the fine'): A speciality of Marennes and Oléron, these oysters are finished in the old salt marshes. The fines stay there a month, 20 oysters to a square metre. An even more deluxe version, the spéciales, are only 10 to the square metre and stay in the saltmarsh twice as long, developing more flesh with a very sweet flavour. The vertes (green fringes) are very prized, retaining traces of a microscopic algae. They are eaten plain or with a drop of lemon juice, a trace of shallot vinegar and bread and butter.
Source: Régal No 38.