Divaricate patterns in pelecypod shells

Shingled divaricate ribs occurring in several unrelated groups of pelecypods are highly functional structures related to specific burrowing processes. However, their obliqueness and ambivalence, seemingly the most aberrant features, are shared by a variety of colour, mineralization, and groove patterns both in the functional and non-functional field. Their basic form is assumed to be less controlled by adaptation and phylogeny than by a common principle of shell growth, the nature of which has still to be found out.