Morphology, palaeoecology, and phylogeny of oysters of the genus Deltoideum Rollier from the Upper Jurassic and Lower Cretaceous deposits of Siberia are discussed. The diagnosis and composition of the genus are revised. A new endemic subgenus, Boreiodeltoideum subgen. nov., of the genus Deltoideum is described. This subgenus includes two species, D. (B.) praeanabarensis (Zakharov) and D. (B.) anabarensis (Bodylevsky).
