Abstract
The geometry of the commarginal shell sculpture and internal microgrowth pattern in 82 species of the Bivalvia were analyzed both theoretically and empirically. Two major categories were recognized in the geometry of internal microgrowth increments; 1) the regular type, which consists of regularly arranged curves, being mutually parallel in the sections and maintaining their morphology during growth independent of the sculpture pattern, and 2) the undulated type, with undulated microgrowth increments along the folds of prominent sculpture. For the former category, geometric patterns of the sculpture and increments of actual specimens were well reproduced by computer simulations under the condition that the sculpture is originated by mantle extension and shrinkage. In the latter case, mantle bulging or bending appears to produce a plicated shell folding, although sculpture formation cannot be simulated by the model adopted in this study.
