Right and left valves. Bivalves have a right valve and left valve. When the exterior of a bivalve shell is oriented with its hinge and umbo (and beak) upward and its commissure (valve-opening margin) downward, and the umbo (and beak) farther away from you (dorsal-anterior side of the shell), the right valve is on your right side, and the left valve is on the left side. In many bivalves, the umbos and beaks are angled toward the anterior side of each valve (Cox and others, 1969; Carter and others, 2012).
Exterior parts. The umbo is the backward (dorsal)-narrowing, raised projection of the valve above the hinge line. The beak is the most rear part of the umbo where growth lines meet at the rear of the valve, generally extending slightly over the hinge. The umbo, beak, and hinge are on the dorsal (back) side of the valves in most bivalves, although positions vary depending on shell shape. Bivalves open along the opposite ventral (front) side, along the commissure. At right angles to the dorsal and ventral sides are the anterior and posterior sides. In life, the anterior side is the forward-facing part of the shell, and the posterior side is the rear of the shell (Cox and others, 1969; Carter and others, 2012). In some fossil bivalves it is relatively easy to determine anterior, posterior, dorsal, and ventral sides; in others it is more difficult without seeing them in life position.
In many bivalves, the shell of each valve is depressed inward on either side of the umbo and beak along the hinge. The depression on the posterior side of the umbo is usually the longer and narrower of the two depressions, and is called the esutcheon, or posterior cardinal area. It is where a hinge ligament is located in life. The depression on the anterior side of the umbo is shorter and often wider. It is called the lunule or anterior cardinal area. In more symmetrical bivalves (like scallops), the depression and hinge ligament are directly behind the umbos and beaks, and the depression is called the cardinal area (Cox and others, 1969; Carter and others, 2012). The shape and position of these features is important in the description of some fossil bivalves, and can aid in determining dorsal, anterior, and posterior sides of a fossil shell.
In some bivalves, part of the shell along the hingeline projects outward like a wing, which is termed an auricle. Auricles are common in scallops and scallop ancestors. Sometimes they occur on both sides of the umbo, and sometimes they are significantly longer on the anterior side, than the posterior side of the umbo. In other bivalves, such as some mussel shells, a long ridge may project toward the ventral-posterior margin of the valve from the umbo, which is termed a posterior ridge.
The outside of fossil bivalve shells may be ornamented with various features such as concentric growth rings, thick transverse ridges termed ribs, thin transverse ridges termed costae, and bumps or spines, similar to modern bivalves. Spines themselves are generally not preserved as fossils, but broken bases of spines may remain. Of course, most exterior ornamentation is not preserved on the inside of valves (ribbing is sometimes an exception), so will not be preserved on internal casts and molds of bivalves, which are a common type of bivalve fossil.
Internal parts. Many features can also be preserved on the inside of bivalve shells, where the internal surface of valves is preserved. Some bivalves possess hinge teeth (cardinal and lateral teeth) along their hinge, while others are only connected by ligaments. The shape and position of hinge teeth (and associated grooves on the opposite valve) are important in the identification of some bivalves.
Ligament connection areas can sometimes be delineated, although the ligaments themselves are not preserved as fossils. Muscles also are not preserved in fossils, although muscle scars are sometimes preserved (blue dashed lines in examples). In most bivalves, the posterior muscle scar is larger than the anterior scar. The posterior side of the bivalve is where siphons are located for bivalves with siphons. Siphons are fleshy tubes which extend from inside the shell to allow water and food into the shell, and water and wastes out of the shell. Siphons are not preserved as fossils, but a bend in the pallial line, called the pallial sinus can be preserved. The pallial line, when preserved, is a line along the interior of a valve located toward the ventral margin of the commissure, which marks the area of attachment of the soft mantle. The pallial sinus, when preserved, is marked by a large inward bend in the pallial line (orange dashed line in Little neck clam). The pallial sinus opens outward toward the posterior margin of the valves and marks the position of the inhalant and exhalant siphons in bivalves that have siphons (Cox and others, 1969; Carter and others, 2012).