Читать книгу The Conchologist's First Book - Brown Thomas - Страница 6

Table of Contents

Bivalve shells consist of two parts or valves, connected by a cartilage, and a hinge which is generally composed of teeth; those of the one valve locking into a cavity in the other.

The valves of some bivalve shells are formed exactly alike, and others are very different; the one being smooth, the other rugose; one flat and another convex; and often one is shorter than the other.

The shells of the Mya, Solen, Tellina, Venus, and others, have in general both valves alike, while those of the Spondylus, Ostrea, and Anomia, have in general dissimilar valves. The first of these kinds are called equivalve, and the latter inequivalve.

Equilateral shells, are those whose sides are alike, as in the shells of the genus Pecten. Plate II. fig. 2, and Plate VII. fig. 14. This is also exemplified in the Pectunclus.

Inequilateral valves are shells whose sides are unequal; and of different shapes, as in the Mactra, Donax, &c.

Summit is the most elevated point of that part of the shell in which the hinge is placed. Plate I. fig. 4, i k.

In naming this the summit we do not follow the axiom of Linnæus, but because we consider it more properly the summit of the shell than the opposite extremity.

Base is the reverse of the above, or that part of the shell immediately opposite the summit. Plate I. fig. 6 and 7, d d.

Sides, the right and left parts of the valves. Plate I. fig. 6, c.

Posterior slope is that part of the shell in which the ligament is situated. In viewing the posterior slope in front, the beaks of the shell retire from view. Plate I. fig. 4, i.

Anterior slope, that part of the shell opposite the posterior slope; in viewing it in front, the beaks point to the observer. Plate I. fig. 4, k.

Disk, the convex centre of a valve, or most prominent part of the valve, suppose it with its inside lying undermost. Plate I. fig. 4, o.

Inside, the concave part of a valve. Plate IV. fig. 6, m.

Muscular impression is the impression left on the inside of the valves, by the adhering muscles of the animal. It differs in most shells, according to the shape of the animal, as semi-ovate, round, lunate, elongated, &c. As a specific distinction, it is often of great use; being, with a very few exceptions, alike in shells of the same species. Some shells have only one cicatrix, as the Edible Oyster and Mytilus; others have two, and some few more; the Tellina for example. Plate I. fig. 6 and 8, e e e e.

Lunule. The lunated depressions, situated in the anterior and posterior slopes. In different species of Venus they are prominent characteristic marks, often of much service in ascertaining a species. Plate I. fig. 4, b b.

Ligament perforation. The circular aperture, or perforation through which the ligament passes; by which the animal of the Anomia attaches itself to stones and other marine extraneous bodies, it is in general situated in the flat valve, though there are a few exceptions to the contrary. Plate I. fig. 7, m.

Hinge is the point at which bivalve shells are united; it is formed by the teeth of one valve inserting themselves between those of the other, or by the teeth of one valve fitting into the cavities or sockets of the opposite one.

It is on the peculiar construction of the hinge that the generic character of bivalve shells is principally founded, together with the general contour of the shell. Plate II. fig. 1 and 3, q q q.

Teeth of the hinge. Upon the number and relative situation of the teeth principally depend the specific distinctions; they are of various forms, and very differently placed; some are single and large, others numerous and small, orbicular, spatuliform, laminated, &c. Some hinges have no visible teeth, and are termed inarticulate.—When a primary tooth has a groove or hollow in its centre, it is called complicated. Plate IV. Fig. 6, e; those with few teeth are termed articulate. Plate IV. fig. 1 and 12; and those with many teeth multiarticulate. Plate IV. fig. 2, 3, 4, &c. Plate I. fig. 6, and 8, g g g g.

Primary teeth are those teeth in general situated in the centre of the hinge, and are for the most part broad, large, and distinct, often elevated; and in general are inserted in a cavity in the opposite valve. They however differ very much in some shells, but may easily be distinguished. Plate IV. fig. 1, 3, and 7, a a a a a. These are also termed the Cardinal Teeth.

Lateral Teeth, Plate IV. fig. 10, are teeth which diverge from the Umbo, and are in general long and flat, often double and divided by a groove or hollow. Plate IV. fig. 2, 4, 6, and 7. b b b b.

Double Teeth, Plate IV. fig. 1, c c.

Incurved Teeth are those which are bent round, as in the single tooth of the Solen. Plate IV. fig. 8, f f.

Recurved Teeth, are those which are bent backwards, as in the hinge of the Panopea and Spondylus.

Middle Teeth, Plate IV. fig. 4, d d. These are also termed Cardinal Teeth.

Numerous Teeth, are those small upright teeth, set in rows, of which the hinges of all the species of the genus Arca are formed. Plate IV. fig. 10, k k.

Cavity of the Hinge. The hollow depression in which the ligament of the Ostrea is situated, generally of a triangular form. Plate IV. fig. 11, g.

Ligament of the Hinge, or cartilage, is that flexible fibrous substance by which the valves are united, and the hinges kept in their proper places; generally situated under the beaks of the shell. Plate II. fig. 1, 3, and 4, l l l; Plate IV. fig. 3, n.

Beak is the extreme point of the summit of bivalves, which in many species turns spirally downwards or to the one side, as in some species of Venus, &c. From this circumstance it is seldom the highest part of the shell. Plate II. fig. 5, r r.

Seam. When the valves are closed, the line of separation between them is so called. Plate II. fig. 6, t t.

Umbo. That part situated immediately under the beak. Plate I. fig. 6, w; and Plate II. fig. 4, w.

Ears. The processes on each side of the beak, in most species of that division of Ostrea, called Pecten; some have one ear very large, and the other small; and some are scarcely observable on one side. Pl. II. fig. 2, h h.

Superior Ear. Plate IV. fig. 11, h.

Inferior Ear. Do. do. i.

Margin. The extreme edge of the whole shell, or the circumference of either valve, all round. Plate I. fig. 4, p; and Plate II. fig. 1, p.

Crenulated margin. That fine notched edge of shells, which unite into notches in the opposite valve, as genus Donax. Plate IV. fig. 6, m m.

Striæ are fine thread-like lines, generally on the exterior surface of shells, and are sometimes both longitudinal and transverse. When the striæ of shells appear indistinct, as if worn out, it is termed obsolete striæ. In some instances the insides of shells are striated; for example, the Fasciolaria Tulipa. The character of the striæ is often of much use in distinguishing species. Plate II. fig 2, f.

Right Valve is that valve which, when viewed with the inside uppermost, has the anterior slope pointing to the right hand. Plate I. fig. 8; and Plate IV. figs. 5 and 6, B.

Left Valve. The opposite of the above; the anterior slope points to the left hand, when viewed from the inside. Plate IV. figs. 3, 6, 7, 9, &c.

Length of the Shell is taken from the ligament, or the beak, to the opposite margin. For example, Mytilus: it is longer than it is broad, and the Solens are broader than long. Plate I. fig. 5, u u; and Plate II. fig. 4, v v.

Breadth is measured from the most extreme edge of the anterior and posterior slopes, being in a contrary direction from its length. Many shells are broader than long, such as most of the Myæ, Solenæ, Tellinæ, &c.; and the Mytilus, Ostrea, Pinna, &c., are in general longer than broad. Plate I. fig. 5, v v. Plate II. fig. 4, v v.

Byssus, or beard, is an appendage composed of filaments of a silky texture, by which some of the Bivalves fasten themselves to their beds, such as the Mytilus, Pinnæ, &c. Plate II. fig. 6, s.