Part 7

Ligament and Ligamentary Cartilage of the Hinge.

These two distinct substances have been described by many writers as though, composing the same mass, they were of one substance; but the difference may very easily be explained. The true ligament is external, being fixed on the edge of one valve behind the umbones, and passing over in an arch to the corresponding edge of the other, very correctly retaining the name of ligament, because it serves the purpose of binding the two together. The thick, elastic substance, which Mr. Gray names the cartilage, is sometimes found in connexion with the ligament, so as to form one mass with it, although it is always separable and placed within it: it is sometimes placed quite within the shell, and separated from the ligament, in a pit or hollow formed for its reception in the hinge lamina, near the centre. It is found in both valves, and being elastic, the portion in one valve presses against that in the other, so as to keep the valves apart, unless voluntarily closed by the adductor muscles of the animal. The ligament is sometimes spread over an external area, as in Arca, while the cartilage is placed in several grooves of the same area, beneath the outer covering.

Hinge lamina, Teeth and Fulcrum of the Ligament.

In a great variety of cases, there is a thickening of the substance of the shell within, under the dorsal margin; this is named the hinge lamina. It is sometimes merely callous; but in many cases it has raised teeth in both valves, those in one valve entering into corresponding cavities in the other. Those which are placed immediately below the umbones, and seem to take their rise from beneath them, are called cardinal teeth; those at a distance from the umbones, which are seen to lie along the upper margin of the shell are named lateral teeth.

When the cardinal teeth terminate in a double point, which is not unfrequently the case, they are said to be bifid. The lateral teeth, in various species, are distinguished as terminating near to, or at a distance from the umbones. In the Nuculæ and Arcæ there is a row of teeth placed across the hinge lamina. In which case, the lateral cannot be distinguished from the cardinal teeth.

Muscular Impression.

Lamarck divides the Bivalve shells into two general orders; the first is named "Dimyaria," having two adductor muscles; and the second, "Monomyaria," having but one. These adductor muscles are used for the purpose of drawing the valves together, being composed of contractile fibrous gristle, fastened firmly to the inner surface of each valve. The place where they are thus fixed may be seen when the animal is removed, by depressed areas, which are generally pretty well defined, and are named muscular impressions. Where there is but one adductor muscle, there will be but one of these impressions near the centre of each valve, but in the Dimyaria, where there are two, the impressions are seen, one on the anterior, and one on the posterior of each valve, just below the hinge lamina. They are sometimes complex, that is composed of several portions in a group; but in general, they are simple and well defined.

They are also described as large or small, in proportion to the size of the shell; regular or irregular in form. The animal is attached to the inner surface by the fibrous portions of the mantle, which creates a linear impression or cicatrix, commonly described as the palleal impression, or muscular impression of the mantle. It runs near the ventral margin from one muscular impression to the other, sometimes in a smooth continuous line or band, and sometimes in an interrupted series of small impressions. Near the point of union with the posterior muscular impression, there is sometimes a more or less considerable winding inwards towards the centre of the shell, and back again towards the point of union. This is named the sinus, and is distinguished as being angular or rounded, large or small, according to the species. When it enters towards the centre of the shell in a tongue-shaped outline it is said to be ligulate. Where it exists it affords a certain index to the posterior side of the shell; as it is the region through which the excretory tubes pass.

Umbones.

These are the prominent points of the dorsal edge, where the growth of the shell commenced, and are called beaks, by some English writers. In some instances they are close to each other; in others they are rendered distant from each other by the intervention of areas in the hinge, as in Spondyli, &c. In Pectunculus they are straight; in Venus curved towards the anterior margin; in Isocardia, spiral; in Chama, decumbent; in Diceras, free. In shells subject to external corrosion, the process commences at the umbones.

GENERAL CHARACTER OF BIVALVES.

When the breadth is spoken of, the distance between the most convex parts of both valves, when closed, is intended; but when an expression implying thickness is used, it refers to the substance of each valve: it is important to bear this in mind, as many persons have been misled by descriptions in which the distinction has not been attended to. Glycimeris (fig. 67 in the plates) is a thick shell, but Anatina (fig. 69 in the plates) is a broad one.

Regularity.

A great number of Bivalves are extremely regular in their form. These are generally locomotive, and consequently free from those obstructions in growth occurring to stationary shells, which being confined in a particular position, or to a particular spot, modify their shape according to the substance with which they come in contact, and thus become irregular. This is generally the case with shells which are attached to submarine substances, such as Spondyli, Oysters, &c.; and the degree of irregularity will depend upon the extent of surface involved in the attachment. In the case of fixed shells, the attached valve is usually termed the under valve, and the other which moves freely upon the hinge, is termed the upper valve.

Form and Proportions.

Bivalves are said to be equivalve when the two valves correspond in extent, breadth, and thickness; and of course inequivalve in the contrary case. They are equilateral when a line drawn from the umbones to the ventral margin would divide the shell into two nearly equal parts; and of course inequilateral in the opposite case, which occurs in the great majority of instances.

A Bivalve is said to be compressed, when the distance is small from the most prominent part of one valve to that of the other. It is cylindrical when lengthened, and more or less rounded in its breadth, as in Lithodomus (fig. 161 in the plates). It is cordiform when the shape presents a resemblance to an imaginary heart, as in Cardium cardissa (fig. 122 in the plates), and in the Isocardia (fig. 126 in the plates). It is linguiform when it resembles a tongue in shape, as in Vulsella (fig. 185 in the plates); rostrated when it protrudes at either extremity, and terminates in a kind of point, as in Sanguinolaria Diphos (fig. 99 in the plates); truncated when it ends in a square or angle, as if cut off; an example of which may be seen in Solen (fig. 60 in the plates).

Other Bivalves are distinguished as being auriculated, having processes flattened and expanded on either side of the umbones, as in Pecten (cut, fig. 82). When there is one of these on each side of the umbones, it is bi-auriculated; when only on one side, it is uni-auriculated. When the expansion is very broad, as in Unio alatus (fig. 142 in the plates), and in the Hammer Oyster (cut, fig. 83), the term alated is used.

With regard to these alated species of Uniones, it is necessary to observe that they are also "adnate," as it is termed; the two valves being joined to each other by the dorsal edge of the expanded parts, and united so completely in substance with each other, that they cannot be separated without being broken. Many other terms are used to express difference in Bivalves, but being generally applicable to Univalves and Multivalves, as well as to them, they will be found explained at large in the alphabetical part of the work.

MULTIVALVE SHELLS.

These are of three different kinds; first, the "dorsal," as they are termed by Linnæus, because they form a ridge in the back of the animal. They are composed of eight pieces, or separate valves, placed in a longitudinal series, being joined to each other by inserted lamina, and named Articulata by De Blainville, on that account. The genus Chiton is the only example of this kind of Multivalves.

The second kind, M. De Blainville terms the lateral bivalves, the pieces being placed in pairs on each side of the animal; these compose the "Pedunculated Cirripedes."

They differ considerably in the number and arrangement of the valves; the small ones, which are found near the peduncle in some species, are sometimes termed accessary valves; those which form the edge through which the bunch of Cilia protrude, are termed ventral, and those on the opposite side dorsal. The extremity joining the peduncle is the basal, or anterior; and the upper extremity is the apsiral, or posterior. The peduncle is the medium of attachment to submarine substances, to which this well known tribe of shells adhere.

The third kind are termed coronular by De Blainville, and compose the order Sessile Cirripedes of Lamarck; they consist of a number of valves placed against each other side by side in a circle, supported on a plate, or tube, or cup, and closed by an operculum composed of two or more valves.

The basal support is sometimes thick and flat, sometimes forming an elongated tube, and sometimes hollowed out into a cup. In other species it is altogether wanting. The operculum always consists of more than one piece, generally of two pairs: they are either articulated to each other by serrated edges, and placed against each other conically, as in Balanus, or they lie flat in two pairs against each other. Through the ventral pair the cirrhi protrude.

The parietal valves, composing the principal part of the shell, vary in number, form and position. The anterior valves are placed on the same side with the cirrhi; the posterior, those on the opposite side; and those which remain between on each side are the lateral valves. In many cases, particularly in Balanus, each valve is separated into the prominent and depressed areas, and the inserted lamina. In some instances, the parietal portion is formed by a single rounded piece.

In the accompanying cut (87), the prominent areas are distinguished by the letters pr, and the depressed areas by r; the posterior valves of the operculum are marked p. o., and the anterior a. o. The basal valve (fig. 88) belongs to a Balanus. Fig. 89 is an Acasta, the cup-shaped base of which is represented at fig. 90.

In the foregoing explanations we have omitted many of those general terms which, relating to external characters, are applicable to shells in almost every division of the system. It may be as well, however, to enumerate a few of them in this place, although they are explained under their respective letters in the alphabetical part of the work.