FOSSIL SHELLS.

That the student may understand what follows, it is necessary for him to become acquainted with the principles on which shells are arranged, and a few of the terms by which the different parts of a shell are are denoted.

Shells, in their recent state, are composed of carbonate of lime, mixed with a little animal or gelatinous matter. In their fossil state, the gelatinous matter is seldom present, though sometimes a small quantity has been detected.

CONCHOLOGY.

Conchology is the seience which treats of the structure, arrangement, and properties of shells. Shells are inhabited by testaceous animals, and to which they are only partially attached. Crustaceous animals are confined entirely within their coverings, each limb, or member being invested by its own peculiar shield, as in the lobster and crab. Many of the testacea are fixed by an attachment to other substances, as the oyster and muscle; while others have the power of crawling along the bottom, or of moving through the water, as the unio, (fresh water clam,) and the scallop. The animals which inhabit shells are called mollusca or molluscous animals, but the classification depends, not on the habits or form of the animal, but on the form and other properties of the shell.

The Linnæan system of conchology, which is the most simple of any that has been proposed, divides shells into Multivalves, Bivalves, and Univalves.

By valve, is here meant any single piece of shell, which forms the habitation or part of the habitation of a molluscous animal. Any shell formed of more than two pieces is a multivalve. Bivalves consist of two distinct pieces, and univalves of a single piece.

Fig. 68.

1. MULTIVALVES.

This is much

the smallest class, but contains some beautiful shells. The genus Lepas, which contains the common barnacle, fig. 68, belongs here. The Chiton or coat of mail is another member of this class. The generic description of Lepas is, "Shell multivalve; affixed at the base; valves unequal, erect.These shells are chiefly parasitic, being attached to extraneous substances, often to ships, pieces of wood, whales, &c. The generic characters of this class are derived from the number and situation of the valves.

2. BIVALVES. This class includes all such shells as are composed of two pieces only, whatever their forms or dimensions may be; and in these respects, the species differ exceedingly. The two valves of the Chama gigas sometimes weigh 500 pounds, and from this, there are all grades of size down to that of a grain of sand.

f

Fig. 69.

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Bivalve shells, when their valves are similar in size and form are said to be equivalved, if not similar, inequivalved; when the anterior part agrees in form with the posterior, they are said to be equilateral, if not, inequilateral. The valves are connected at their base by a ligament, with or without a hinge, the ligament being placed externally or internally. The belly, a, fig. 69, is the most tumid part; the disk b, is that part between the belly and the margin c, which is considered to refer to the external side, or as it may be termed, when the shell is placed on its base, the upper side: then the umbones, (eminences) d, are beneath the hinge, and terminate in the points or beaks, e, which are incurved, reflected or ear formed. The beaks are frequently, in particular shells, accompanied by two external impressions, one of these, the

corslet, f, is on the anterior surface, and is separated from the disk, generally by a ridge, an angle, or a sunken line; and is often distinguishable by its difference of color; it is sometimes spinous, carinated, lamellated, &c., but is more generally smooth, when it is said to be naked. The other impression, called the lunule, g, is placed at the bottom of the posterior surface; it is variously shaped, oval, oblong, lanceolate, &c. The two pieces, forming the shell, are called the right and left valves. The shell being placed on the hinge with the anterior side forward, that is considered as the right valve, which answers to the left hand, the other being the left valve. The length of a bivalve is from the umbones to the margin opposite; and the width or breadth, from the end of the anterior, to that of the posterior'margin. Hence many shells are broader than they are long. Those whose length exceeds their width, are called longitudinal, and those whose width exceeds their length, are called transverse shells. Shells are distinguished by the appellations free, when they are capable of moving, and fixed when they adhere to other bodies.

Bivalves are divided into three orders, depending on the mechanism of their hinges.

Fig. 70.

First. Those which are furnished with internal teeth at the hinge, but which are not inserted into the opposite valves, as in fig. 70. The genus Mya, (to which belongs the common long clam,) and Solen, (razor shell,) are examples.

To the Mya genus, belongs the Pearl Gaper, (Mya margaritifera,) a beautiful shell with a pearly lustre, and which occasionally produces pearls of great value. It is found in the large rivers of the northern latitudes, and is not the shell which is the object of the regular pearl fishers.

is

To this genus also, belongs some shells in considerable request among collectors; but on the whole, the genus not remarkable for the beauty of many of its species.

· Fig. 71.

Second. Shells which have their teeth inserted into their opposite valves, fig. 71. To this order belongs the Cardium, (heart shell,) and the Venus; one species of which, is well

known in our markets, under the name of round clam, and which are taken in great abundance on the shores of Long Island, and sold as an article of food..

Third. Shells, having a hinge, without teeth, as in the well known shells, the oyster and scallop.

The generic distinctions of the bivalves, depending entirely on their teeth, and their genera in the Linnæan system, amounting to only thirteen in number, they are easily distinguished from each other. The species depending on the forms and markings, are not so readily distinguished.

3. UNIVALVES. This is much the largest class, and contains a great proportion of the shells which collectors seek after with so much avidity, and many of which are exceedingly beautiful in their forms and colors.

Univalves differ greatly from each other, in form, size, and coloring. Like the bivalves, their different parts are distinguished by peculiar names, which are applied in scientific descriptions, and by means of which, conchologists are enabled to understand each other.

Only the most necessary and common of these terms can here be explained.

The univalves are distinguished chiefly by the form, size and direction of their apertures, but sometimes by the conformation of the shell.

The univalves are exceedinly numerous, of a great variety of forms; and some of them remarkably beautiful, both in shape and color. The names of the several parts of a univalve will be distinguished by the annexed cut.

Fig. 72.

The spire, a, in the univalve, fig. 72, is formed by the union of the turns, or whorls, b, which are counted by reckoning the lower turn, containing the opening below, as the first, and counting on the same line to the top of the spire. The turns in most shells, go from the right to the left; when they pass from left to right, which rarely happens, the shell is said to be reversed. The line passing round the shell like a screw, and at which, the whorls are united to The whorls are plain, grooved, crenulated, crowned with points, &c.

each other, is termed the suture.

The back of the shell, c, is the external, tumid part on the opposite side, and above the aperture. The body consists of the whole of the tumid part, c, which forms the first whorl. The opening, or aperture, e, is circular, oval, angular, &c. and it is often the form of this part which determines the genus of the shell. This opening terminates in a groove or notch, which is either straight or turned to the right or left, or backwards. When the opening is longer than wide, it is said to be longitudinal; and when wider than long, transverse. The edge or margin of the opening is divided into right and left lips. The right, or outer lip, d, reaches from the body, or first turn of the shell to the base. The left lip, g, is on the other side of the opening, and is of small extent in those shells, the openings of which are entire. This opening is filled with a body composed of shell, or cartilage, which is attached to the animal, and with which he can close the opening at pleasure, by drawing it in. This is termed the operculum. The little white bodies popularly called eye-stones, are operculums.

Univalves are distinguished into two kinds.