Oyster, or Ostrea, L. a genus of shell-fish, comprising thirty-one species, which are distinguished chiefly by the peculiar formation of their shells.- The principal of. these is, the Common Oyster, taken at the mouth of rivers, in clear waters, on the. eastern coast of Britain. Among the most esteemed" for their delicious flavour, are the Maiden and Colchester Oysters, caught in the Pent-Burnham, Maiden, and Colne waters ; or near the mouth of the Thames, which last are said to rival those of Colchester.

Oysters cast their spawn in the month of May, when they become subject to a periodical affection; the male-fish, having a black substance in the fin, is black-sick; and the female oyster, from a milky-juice in its fin, is said to be white-sick : in June and July they begin to recover ; and are in August perfectly sound.—They are saltish in the pits, more saline in the beds or layers, and very salt in the sea.

These shell-fish should be fresh, tender, and moist 3 as the want of fresh water renders them hard, bitter, and unpalatable.- Epicures give the preference to such as are edged with a small brown fringe, or beard, and which they erroneously suppose to be females. It is equally absurd to conclude, that the fine green observed in oysters taken from artificial beds, is the effect of copperas; as this substance, or a solution of it, is inevitably fatal to all fish.

Oysters are esteemed as excellent food, and are eaten both raw and dressed, in various ways: in a fresh state, however, they are doubtless preferable ; for, by cooking, they are in a great measure deprived of their nourishing jelly, and of the salt-water which promotes their digestion in the stomach. Hence raw oysters may be used with equal advantage by the robust, the weak, and the consumptive. Independently of the nutritive effects peculiar to this shell-fish, it general.y tends to open the bowels, especially if a certain quantity be swallowed at one meal: hence to persons of a costive habit, they afford a dietetic supper.

The shells of the oyster, like those of other crustaceous fish, are composed of calcareous earth, and animal glue. They possess no medicinal virtue superior to common lime-stone or chalk ; but, by calcination, they yield a quick-lime, which is perfectly free from any metallic or other tossile substance; and being less permeable to water, when mixed with sand, it is better calculated for the plastering of walls in damp situations. Hence the Dutch prepare their excellent mortar generally of marine shells burnt into lime ; which makes a most durable cement. The great importance of this fact, in point of health and economy, deserves equal attention ; so that the immense quantities of oyster-shells annually thrown away in London, Bristol, and other populous places, might easily be converted into a very useful shell-lime.