The marine faunas from Patagonia, which are in need of a comprehensive revision, are particularly important in the understanding of the origin and evolution of the marine biota in the southern hemisphere during the final breakup of Gondwanaland during the Late Cretaceous–early Tertiary. The Eocene Río Turbio Formation in southern Patagonia (Argentina) contains a unique fauna of marine mollusks. This paper describes 35 species of bivalves from that unit. Nine of them are new: Nucula (Leionucula) guillermensis, Yoldia (Calorhadia) patagonica, Atrina rioturbiensis, Electroma patagonica, Venericardia (Venericor) carrerensis, Crassatella brandmayri, Solena (Eosolen) hunickeni, Pholadidea frenguellii, and Periploma (Aelga) primaverensis. In addition, 26 other species are also described, though their exact systematic relationships cannot at present be accurately established.
