Hinge.
Fig. 73, l t, lateral teeth; c t, cardinal teeth; c, cartilage under the ligament; l, ligament; f, fulcrum of the ligament.
The hinge of the shell is on the dorsal margin, and is composed of the various apparatus by which the two valves act upon each other in opening and shutting. It consists of a ligament, which is placed on the dorsal margin, just at the back of the umbones, and unites the two valves together; the cartilage or thick gristly elastic substance, sometimes found close to the ligament, to which it then forms an inner coating, and sometimes received into a pit within the shell. It serves the purpose of keeping the shell open when not forcibly closed by the adductor muscles. An inner layer of shelly matter upon which are placed teeth, and pits to receive them on the two valves reciprocally. Each of these it will be necessary to treat of more at large; observing, at the same time, that in some species of Bivalves these parts may be wholly or partially wanting. Thus we meet with some shells, such as the Muscle, without teeth; and there is the group containing Pholas, &c. the hinge of which is destitute of teeth and ligament, the two valves being kept together by loose cartilages, and by the contracted space in which they are confined.
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.