Oysters are a family of bivalves with rough, thick shells, the Ostreidae, plus some other types which are called 'oyster'. Many species are edible, and are usually served raw. They are also good when cooked. In history, they were an important food source, especially in France and Britain. They used to grow in huge oyster beds, but were "overfished" in the 19th century. Nowadays they are more expensive, so eaten less often.