Bivalves are generally equivalved, which means the two valves are mirror images of each other. Each individual valve, however, is generally not symmetrical across the length of the valves, termed inequilateral symmetry. Some bivalves have equivalved, equilateral symmetry. Uncommon inequivalved bivalves have inequilateral symmetry (Cox and others, 1969; Carter and others, 2012).
Shapes of bivalve shells can be described from the side looking along the hinge line (profile) or looking down on a valve (outline). Because bivalve valves are generally equivalved, outlines are generally more important than profiles for defining their shapes. The shape and structure of the hingeline, ligament area, the position and angle of the beak/umbo area, and the presence and angle of posterior ridges are also important in describing some bivalve shapes.
Bivalve profiles. Bivalves usually have biconvex profiles, which means both valves are outward-curving or arching relative to each other. The amount of convexity (compressed vs. inflated) is important for differentiating some bivalves, although compaction during burial and fossilization can influence relative convexity. The width of a bivalve is a measure of the profile (across both valves), essentially between the points of maximum convexity of both valves (Cox and others, 1969; Carter and others, 2012).
Bivalve outlines. Outline shapes are described by the relative geometry, height and length of the valves. Height is the direction from the dorsal (umbo side) to ventral side of the valve. Length is the direction across the valve from the posterior (front) to anterior (rear). A wide range of geometric shape terms are used to describe bivalves (e.g., circular, oval, etc). Additional shape adjectives (e.g., teardrop, spatulate, etc.) may also be used. In some cases, where a fossil is in the same order or family of a living bivalve with a distinctive outline shape, the modern order or family name may be used as an adjective to describe the outline shape. For example, a shape or outline described as “pectinid” would indicate a bivalve with the general outline shape of the genus Pecten, the modern scallop; a mytilid outline would be a bivalve with the outline of the family Mytilidae, meaning like a modern mussel.
It is important to recognize that outline shape terms are based on loose, whole specimens. How a fossil is preserved can influence its relative appearance. Shells preserved in rock matrix may only be partly exposed on the outside of the rock, which may mask their true outline. In some shales, shells may be compacted and flattened, or fragmented, which can also change their original profile and outline shape.