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Savazzi, E. & Yao, P. 1992 04 15: Some morphological adaptations in freshwater bivalves. Lethaia, Vol. 25, pp. 195–209. Oslo. ISSN 0024–1164.

Several freshwater bivalves possess peculiar shell morphologies. An extension of the postero-dorsal shell margins above the hinge line evolved convergently in several unionids. This extension supplements the opening momentum of the ligament, but must be broken off periodically in order to allow further shell growth. Arconaia and Cuneopsis have evolved twisted commissure planes, comparable to those found in unrelated marine bivalves. In marine forms, byssus is believed to have played a fundamental role in the evolution of shell torsion. However, the twisted Unionidae do not possess a byssus in the adult stage, thus forcing us to re-evaluate our ideas on the adaptive value and evolution of shell torsion. Solenaia oleivora is apparently incapable of reburrowing and of retracting its foot within the shell. The foot may be functional as an anchor, and is perhaps involved in chemosynthesis by storing sulphur extracted as sulphide from the surrounding sediment. Other adaptations of freshwater bivalves include selective thickening of portions of the shell that enhance its stability, permanent anterior and posterior gapes, and oyster-like morphologies and shell structures. *Functional morphology, constructional morphology, burrowing, shell torsion, Mollusca, Bivalvia, Unionacea, Recent, Quaternary, People's Republic of China.