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Recently, a species ofOrthoceratiumwas collected in Greece that differs morphologically from the European species commonly presumed to beOrthoceratiumlacustre(Scopoli, 1763). Verification of the identity of the Greek species through comparison with 460 specimens ofOrthoceratiumfrom 17 West Palaearctic and one Afrotropical country, and examination of existing type material, revealed that the species recognized asO.lacustrein northwestern Europe for over 250 years is actuallyO.sabulosum(Becker, 1907), the other known species in the genus, which was originally described from Tunisia. Although the types ofO.lacustrehave been lost, a comparison of the distribution ranges of both species in Europe provided evidence that the species collected in Greece is conspecific withO.lacustre. Both species have distinct distributions in the West Palaearctic, withO.lacustrelargely restricted to the northern border of the Mediterranean basin, andO.sabulosummore widespread, occurring in northwestern Europe, the western, southern, and eastern Mediterranean, the Middle East, and the Afrotropical Region (Tanzania). Both species are redescribed and fully illustrated, a neotype is designated forO.lacustreand a lectotype forO.sabulosum, and a key to males and females is provided. The misidentifications that lasted for over two centuries are explained by the omission by previous authors to study the type specimens, and inaccuracies in species descriptions and keys.