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The genus Ontaria Clarke, 1904 includes a group of small, almost circular bivalves that occur abundantly in the Mid- and Late Devonian pelagic facies. These bivalves are characterised by their straight hinge line, small and, central beak, as well as commarginal rugae (if present). Based on an extensive review of type material and additional samples as well as a comprehensive literature review, this study identifies three valid species Ontaria suborbicularis Hall, Ontaria concentrica von Buch, and Ontaria iniquistriata Beushausen. Thirteen Ontaria species are either not valid or have been synonymised, and three species that were previously included in Ontaria do not belong to this genus. Representatives of Ontaria first occur in the late Eifelian and become abundant in the Frasnian and Famennian. Ontaria last occurs in the late Famennian before the Devonian/Carboniferous boundary. This genus has been identified as a representative of the family Cardiolidae previously reported only from the Silurian, making Ontaria the first report of a Devonian cardiolid taxon. Ontaria inhabited low-energy, generally well-oxygenated, off-shore environments. The overall shell morphology, small size, and the presence of a small siphon supports the idea that Ontaria lived semi-infaunal in/on muddy substrates. [ABSTRACT FROM AUTHOR]