forate the sand or mud at the bottom, where many of the species are caught for food, and others for the pearls which are formed within their shells. Some few of the species perforate and live in lime-stone, in the same manner as the pholades.

THE PEARL-BEARING MYA*.

The rivers Tay in Scotland, and Conwy in Wales, are each celebrated for producing, in great abundance, the Pearl-bearing Mya. Some of the shells, in each river, contain good pearls, but fine ones are by no means numerous. Few of them have any lustre; some are round, oval, or elongated and cylindrical; and others are hemispherical, and somewhat resemble buttons.

The origin of pearls in these shells has been thus accounted for. The Mya which produces them is attacked by several kinds of enemies. Of these, there is one which penetrates into the inside, by working a longitudinal passage between the different laminæ that compose the cover. This small channel, after extending about an inch and a half in length, doubles back in a line parallel to the first, and separated from it by a thin partition of shelly matter. These two parallel lines discover the direction of the worm in entering and returning. This is also distinguishable on the surface, by two small holes close to the edge, and in general near the mouth of the shell. As these small channels are excavated in the part nearest to the mother-of-pearl, or

The hinge is furnished with a strong, thick, and broad tooth, not inserted into the opposite valve. The animal is an Ascidia.

* DESCRIPTION. This shell is about two inches long, and five broad, oblong, and somewhat narrower in the middle than at the ends. It is covered with a dark-coloured, rough epidermis, or skin, except on the protuberant parts near the hinge. There is a single tooth in one of the valves, which locks into a forked one in the other.

SYNONYM. Mya margaritifera. Linn.

silvery internal coat, the pearly juice soon extravasates, and protuberances are formed in that direction. The cylindrical bodies thus formed may be considered as elongated pearls, adhering to the internal lining of the shell. When several worms of this kind penetrate near each other, and unite their labours, the result is a sort of pearly wen, with irregular protuberances, in which the issues of the several passages are easily perceptible.

There are other kinds of worms which pierce the pearl-shell, and form cavities more or less round, in which the juice consolidates into pearls.

It was doubtless this observation that first suggested to Linnæus and others, the idea of making artificial perforations in the shells, and thus forcing them to produce pearls. M. Saint Fond, when in London, saw some pearls which had been brought from China, and which had undergone this operation. The artificial hole in these was filled with a piece of brass wire, rivetted on the outside of the shell like the head of a nail; and the part of the wire which pierced the interior shining coat, was covered with a well-shaped pearl, which seemed as if it had been soldered to the extremity of the wire. It is probable that with the Chinese this is not a discovery of very modern date.

M. Saint Fond was informed of another mode of obtaining pearls. The shell from which the pearl is to be obtained must be opened with the greatest care, in order to prevent the animal from being injured. A small portion of the inner surface is then to be scraped off, and a spherical piece of mother-of-pearl, about the size of a small grain of shot, is to be introduced. This globule serves as a nucleus to the pearly juice, which concretes around it, and after a certain time produces a fine pearl.

With respect to the pearls of the river Conwy, it is reported that Sir Richard Wynne, of Gwydir, presented one to the queen of Charles the Second, which was afterwards placed in the regal crown. An Irish pearl is mentioned by Sir Robert Redding, in the Philoso

phical Transactions, which weighed thirty-six carats, and was valued at forty pounds.

OF THE SOLEN OR RAZOR-SHELL TRIBE*.

These animals in general reside in holes, which they form in the sand at the bottom of the ocean. Their position in these holes is always upright. In situations where they are exposed to the air by the ebbing of the tide, their place is easily known to fishermen, by a small dimple which they leave on the surface. Some of the species live in stone. Nearly all of them are used as food.

THE COMMON AND THE SCABBARD RAZOR-SHELL.

Many of the bivalved shell-fishes have the powers of progressive or retrograde motion, by an instrument that has some resemblance to a leg or foot, and called the tongue. But these animals can, at pleasure, make this tongue assume almost every form which their exigencies require.

Like all the other species of Razor-shells, they are in

The shell is oblong, and open at both ends. Its hinge is furnished with a sharp, reflected tooth, not inserted into any groove in the opposite valve. The animal is an Ascidia.

+ DESCRIPTION. This shell is seven or eight inches broad, and one inch deep. It is linear, and straight; and one of the hinges is two-toothed. The colour is olive-brown, with a conoid ash-coloured mark dividing the shells diagonally. One part is striate longitudinally, and the other transversely.

SYNONYMS. Solen Siliqua. Linn.-Solen Silique. Bosc.

DESCRIPTION. This species is not quite so large as the last. It is linear, straight, roundish, and margined at one end. The hinge has a single tooth in each valve. Its general colour is yellowish.

SYNONYMS. Solen vagina. Linn.-Le Fourreau. Cuvier. -Solen Manche de Couteau. Bosc.

VOL. IV.

capable of progressive motion on the surface; but they dig a hole or cell in the sand, sometimes two feet in depth, in which they can ascend or descend at pleasure. The instrument by which their motions are performed, is fleshy, cylindrical, and situated near the centre of their body. When necessary, the animals can make the termination of the tongue assume the form of a ball. The Razor-fish, when lying on the surface of the sand, and about to sink into it, extends its tongue from the inferior end of the shell, and makes the extremity of it take the form of a shovel, sharp on each side, and terminating in a point. With this instrument the animal cuts a hole in the sand. After the hole is made, it advances the tongue still further into the sand, makes it assume the form of a hook; and with this hook, as a fulcrum, it obliges the shell to descend into the hole. In this manner the animal operates until the shell totally disappears. When it chooses to regain the surface, it forms the termination of the tongue into the shape of a ball, and makes an effort to extend the whole tongue; but the ball prevents any further descent, and the muscular effort necessarily pushes the shell upward, until it reaches the surface. It is amazing with what quickness and dexterity these seemingly awkward motions are performed.

It is remarkable, that the Razor-fish, though it lives in salt water, seems to abhor salt. When a little salt is thrown into the hole, the animal instantly quits its habitation. But it is still more remarkable, that, if the animal be once seized with the hand, and afterwards allowed to retire into its cell, salt will then be strewed in vain, for the fish will never again make its appearance. If it be not handled, the animal, by an application of salt, may be made to come to the surface as often as a person pleases; and fishermen sometimes make use of this stratagem as a means of catching it.

These shells are not uncommon on most of our sandy shores.

OF THE CARDIUM OR COCKLE TRIBE*.

On sandy shores of almost all the known seas, some of the species of Cockle are to be observed. Most of them are found immersed in the sand, at the depth of a few inches. Their size is various, from five or six inches to half an inch in diameter. In a fossil state these shells are by no means uncommon; and species corresponding with some of them inhabit the Indian

ocean.

THE COMMON COCKLET.

All the loco-motive powers of these well-known animals are concentrated in the triangular yellow foot, which is so conspicuous when we open the shells. This foot is not only capable of great inflexion, but also of seizing with its point the glutinous matter which proceeds from it, drawing this into threads, and thereby, in some measure, securing the animals within the sand which they inhabit.

Few of our shell-fish are more common, in inlets and bays near the mouths of rivers, than these. In such situations they are usually found immersed at the depth of two or three inches in the sand, the place of each being marked by a small, circular, depressed spot.

When they open their shells, the entrance into these is protected by a soft membrane, which entirely closes up the front, except in two places, at each of which there is a small, yellow, and fringed tube. By means of these tubes it is that the animals receive and eject the

* The shell is bivalve, equivalve, convex, and in most of the species, longitudinally ribbed. The hinge has two teeth near the beak, and a larger one placed remote on each side, locking into the opposite valve. The animal is a Tethys.

+ SYNONYMS. Cardium edule. Linnæus.-Bucarde sourdon. Bosc.-Le Coque. Cuvier.