Fig. 74. a, anterior; p, posterior; m i, muscular impressions.
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.
Fig. 75, distant; 76, straight; 77, curved; 78, spiral; 79, decumbent; 80, free; 81, close.
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.