The structure of the outer shell layer in radiolitid rudists, a morphoconstructional approach

Radiolitids (Bivalvia, Radiolitidae) built the calcite outer layer of their shell with compact structure as many other bivalves, or with repetitively layered non-compact structure following different patterns: continuous radial ridges, discontinuous radial ridges, normal cellular, cellular with radially elongate cells, discontinuous cellular, and discontinuous cellular with elongate cells. Some special structures may develop in localized parts: radial structures and inner-marginal or radial grooves. All these structures are currently used in the diagnosis of the family and for the characterization of subfamilies. Constructional morphology analyses and re-evaluation of all these structures reveal that they were misunderstood in some important aspects in the bivalve Treatise and that, consequently, the diagnosis of the family has to be emended and their subfamilies reconsidered. All outer shell layer structure related characters are described, illustrated, and interpreted under a constructional approach, some characters are proposed to emend the diagnosis of the family, and two possible clades are preliminarily distinguished.