A substantial lateral asymmetry of the shell is observed in the bivalve genera Claudiconcha (Petricolidae), Jouannetia (Pholadidae), Clavagella and Bryopa (Clavagellidae) that bore in rock or mineralised biogenic substrates. Two general adaptive patterns emerge from this study. (1) Lateral asymmetry is associated with cementation of one valve to the substrate, or is otherwise functional in preventing one valve from moving within the borehole, and (2) it is associated with determinate shell growth and appears only in the adult stage. The only important exception to the latter pattern is Bryopa, which secondarily lost the determinate growth pattern originally present in clavagellids. These features have evolved independently in each family, and differ in several constructional and functional respects among families.
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