Article: Architectural constraints on the morphogenesis of prismatic structure in Bivalvia

Microscopic features of the regular simple, fibrous and non-denticular composite prismatic structures in twenty seven living and eight fossil bivalve species have been examined. Geometrical selection and reclination of prisms caused by competition for space were recognized in many species. The regular simple and non-denticular composite prisms are expressed as a product of aggregated spherulites, while the fibrous prisms as needle aggregates in druses. The main factor which causes geometrical selection of prisms is irregularity ol the settling time among prisms. The density of prisms at the first stage appears to be a principal parameter controlling the degree of geometrical selection. The ratio of accretionary rate to growth rate of prisms at the shell edge mainly determines whether prisms become straight or reclined. Various architectural varieties of the prismatic structure can, therefore, be explained by the geometrical selection model originally developed for inorganic systems.