Pearls are produced only in shells that are bivalves, that is, which have an upper and lower shell closing by a hinge in a manner little differing from the oyster. It is commonly said by the fishermen, that all bivalves in the Red Sea have pearls of some kind in them. This is a very rude and large view of the matter, for though it is true that some excrescences, or secretions, of the nature of pearls, may be found in the bisser, and the large bivalves with which this sea abounds, yet it is well known to all conversant in these matters, that many of the pearl shell itself (I shall not call it an oyster, for it is not one) are found without any pearl or likeness of pearl in them; being, I suppose, not yet arrived to that age when the extravasation of that juice which forms the pearl happens. There are three shell fish in the Red Sea which regularly are sought after as containing pearls. The first is a mussel, and this is of the rarest kind, whether they are now failed in number, or whether they were at any former time frequent, is now unknown. They are chiefly found in the north end of the Gulf, and on the Egyptian side. The only part I have ever seen them was about Cosseir, and to the northward of it, where I must observe there was an ancient port, called Myos Hormos, which commentators have called the Port of the Mouse, when they should have translated it, the Harbour of the Mussel. This fish contains often pearls of great beauty for lustre and shape, but seldom of a white or clear water. Pliny relates this to be the case in the Italian seas, and also in the Thracian Bosphorus, where he observes they are more frequent. The second sort of shell which generally contains the pearl is called Pinna. It is broad and semicircular at the top, and decreases till it turns sharp at the lower end, where is the hinge. It is rough and figured on the outside, of a beautiful red colour, exceedingly fragil, and sometimes three feet long. In the inside it is cloathed with a most beautiful lining called Nacre, or mother-of-pearl, white, tinged with an elegant blush of red. Of this most delicate complexion is the pearl found in this fish, so that it seems to confirm the sentiments of M. Reamur on the formation of pearls, that they are formed of that glutinous fluid which is the first origin of the shell, that it forms the pearl of the same colour and water that is communicated to it from that part of the shell with which it is more immediately in contact, and which is generally observed in the pinna to be higher in colour as it approaches the broadest, which is the reddest end.