The muscles which serve to close the valves leave impressions on the inner surface of each valve. These, when both muscles are present, are known as the anterior and posterior adductor impressions. The impression produced by the muscular edge of the mantle, which curves downwards and backwards from the anterior adductor impression, is known as the pallial line. In shells with only one muscle it is represented by an irregular row of small marks, or disappears altogether (Ostrea). The pallial sinus is produced by the muscles which retract the siphons, and is most marked in those genera in which the muscles are powerful and the siphons large (e.g. Tellina, Mya). It is entirely absent in genera possessing no retractile siphons.
Fig. 184.—Left valve of Venus gnidia L.: A, anterior, B, posterior, C, dorsal, D, ventral margin, AB, length, CD, breadth of shell.
a.m, anterior; p.m, posterior adductor muscle; p, pallial line; p.s, pallial sinus; l, ligament; lu, lunule; u, umbo; c, cardinal teeth; a.l, anterior lateral tooth; p.l, posterior lateral tooth.
Fig. 185.—Right valve of Lucina tigerina L.: A, anterior, B, posterior, C, dorsal, D, ventral margin; AB, length, CD, breadth of shell.
a.m, anterior; p.m, posterior adductor muscles; p, pallial line; l, ligament; u, umbo; c, cardinal teeth; a.l, p.l, anterior and posterior lateral tooth.
Right and Left Valve.—The simplest way of distinguishing the valves as right and left is to hold the shell in such a way that the siphons point towards the observer, and the mouth away from him; in this position the valve to the right is called the right valve, and the valve to the left the left valve. If, however, the animal is not present, it may be remembered that the ligament is nearly always behind the beaks, and that the beaks, as a rule, point forward, thus the right and left valves can generally be named by observation of the beaks and ligament. When the ligament is median to the valves (e.g. Ostrea, Pecten), and the beaks are not curved, the valves may be recognised by noting the fact that the impression of the adductor muscle (in these cases always single) is nearer to the posterior than to the anterior side. In a similar way the pallial impression, which only forms a sinus on the posterior side, furnishes a guide to the valves of Donax, in which the beaks point backward, and of Tellina, in which the beaks are frequently central.
In the fixed inequivalves (e.g. Chama) it is sometimes the right, sometimes the left valve which is undermost, but the fixed valve, whether right or left, is always deep, and the free valve flat. Ostrea and Anomia are always fixed by the left valve.
The lunule is a well-marked area in front of and close to the umbones, usually more or less heart-shaped, and limited by a ridge. Generally it is shallow, but sometimes, as in Dosinia, Opis, and some Cardium, deeply impressed. A corresponding area behind the umbones, enclosing the ligament, is called the escutcheon (Fig. [186]), but it seldom occurs.