Syst. Nat. Gmelin. vi. 3315. D. 2. Mollusca; Cl. 4. Acephala. Ord.l. Testaceae. Cuvier. G.S13. Animal Tethis. Shell bivalve : the valves unequal, and somewhat eared. Hinge toothless, but furnished with an ovate hollow cavity, with lateral transverse furrows. Vulva, or anus, none. Species 105. O. edulis. The common Oyster. Pennant's British

Zoology, iv. 102. t. 62. Officinal. Testae, Lond. The shells.

Syn. Ecailles de Huitres (F.), Austerschaalen (G.), Conchiglia d' Ostrica (I.), Cascara (S.).

This well-known shell-fish inhabits the European and Indian oceans throughout; and is particularly plentiful on the British coasts, which were early famed for producing the best oysters to supply the stews of ancient Rome, in the most luxurious period of its history.1 They are naturally attached to shelving rocks; but for the facility of always obtaining them for the purposes of aliment, they are generally laid down near the shore. They are hermaphrodite, and throw out a spat in spring, which gradually enlarges to a perfect oyster. The nature of the shell in some degree, and the taste and goodness of the fish, depend on the soil of the bed. They are tender and friable on a calcareous bottom, thick and solid on rocks, more-glutinous on marle, and oily and luscious on a slimy bed. The green colour of those fed in pits on the coast of Holland has been supposed to be owing to copper; but it arises from a species of conferva which cover those stagnant pools.1 The best oysters on the British shores are found at Purfleet, the worst near Liverpool. The oyster, when good, is very digestible and nutritious, particularly when eaten raw; and may form an article of food for the phthisical, and convalescents.

When they are sick, which is known by a black substance on the fringe or fin, or by a very milky appearance of it, they are unwholesome. The shells only are officinal.

1 Scrgius Ovata was the inventor of stews for oysters among the Romans. Pliny, lib. xiv. cap. 54.

Qualities.-Oyster-shells consist of alternate layers of carbonate of lime and an animal matter, supposed to be coagulated albumen. When thrown into a fire, they emit a great deal of smoke, the animal matter is destroyed, the carbonic acid dissipated in the form of gas, and pure lime remains.

Medical properties and uses.-Oyster-shells are antacid; but as, in their unburned state, they are less so than chalk, and, when burned, differ in nothing from lime, their retention in the list of Materia Medica is unnecessary.

Officinal preparation.-Testae praeparatae, L.