in zoology. See Ostrea.

Oyster-Catcher. See Haematopus.

Oyster Fishery. See Oyster-Fishery and Ostrea.

Oysters, Fossil. The largest bed that is known of fossil oysters is that near Reading in Berkshire. They are entirely shaped, and have the same substance with the recent oyster-shells; and yet since the oldest histories that mention the place give an account of them, we must suppose they have lain there for a long time. They extend over no less than six acres of ground; and just above them is a large stratum of a greenish loam, which some writers call a green earth, and others a green sand. It is composed of a crumbly Pliocene marle, and a large portion of sand. Under them is a thick stratum of chalk. They all lie in a level bed; and the strata above the shells are natural, and appear never to have been dug through till the time of finding the shells.

The oyster-shells and green earth united make a stratum of about two feet thick; and over this there is a much thicker stratum of a bluish and very brittle clay; but neither has this ever been dug through, except where the shells are found. This is vulgarly denominated pierce-clay, and is esteemed useless. This clay-bed is about a yard deep, and above it is a stratum of fuller's earth, about two feet and a half deep; it is extremely good, and is used by the clothiers. Over this there lies a stratum of a fine white sand, unmixed either with the clay or fuller's earth: this is near seven feet deep, and above it is a stratum of a stiff red clay, of which tiles are made. This is again covered with a little vegetable mould; the depth however of this stratum of tile-clay cannot be ascertained, on account of the unevenness of the hill. These oysters are occasionally found whole, but most frequently in single shells. When they are in pairs, there is generally some of the green land found within them: they seldom stick very fast together; so that unless very carefully taken up, it is not easy to preserve them in pairs.

Oyster-Shells, an alkali far more powerful than is generally allowed, and are in all probability much better medicines than many of the more costly and pompous alkalis of the same class. The proof of alkalis is in their solution by acid spirits; and Mr Homberg found, that they dissolved far easier in acids of nitre and sea-salt than either pearls or coral, or indeed than any of the rest. This he supposes to be owing to their containing in the body of the shell a large portion of sal-sulphur, which is easily perceived upon the tongue, and which keeps the whole influence of the shell in a sort of half dissolved state. These shells are found to produce very sensible effects on the stomach, when it is injured by acid humours; and Mr Homberg thinks, that this easiness of solution is a great argument for their good effects, and that the quantity of sal-sulphur which it contains, contributes not a little towards it; for we are not to look upon that as a salt merely, but as a salt of a peculiar nature, formed of sea-salt by the organs of the animal, and the several fermentations it undergoes in the body of it, in the same manner as the nitrous and other salts of the earth cease to be nitrous, &c. whenever they become blended with the juices of plants, and form with them a salt peculiar to that plant; which is evidently the case as far as respects this salt, it being plainly of a more penetrating taste, and of a different smell, from the salt left by the sea-water between the several external scales or flakes of the shell. OZENA shell. Oyster-shells being thus found by Mr Homberg to be a very valuable medicine, and as one of the common methods of preparing them is by calcination, which, he observes, considerably impairs their virtues, he gives the following method of preparing them for taking inwardly, which he himself always used. Take the hollow shells of the oysters, throwing away the flat ones, as not sufficiently good; make them perfectly clean, and then dry them in the sun; when they appear dry, beat them to pieces in a marble mortar: they will still be found to contain a large quantity of moisture; lay them therefore again in the sun till perfectly dried, and then finish the powdering them, and sift the powder through a fine sieve. Give 20 or 30 grains of this powder every morning, and continue it three weeks or a month. See Chemistry, p. 187.

OZENA, a foul and malignant ulcer of the nose, distinguished by its factor, and often accompanied with a caries of the bones of the nose.