Type material of the four nominal Palaeocene-Eocene species of Heterodontus from the North Sea Basin, H. lerichei Casier, 1943b, Heterodontus vincenti (Leriche, 1905), Heterodontus wardenensis Casier, 1966 and Heterodontus woodwardi Casier, 1946 is restudied, based on a very detailed analysis of H. vincenti specimens recovered from the Ypresian of Egem (Belgium) and on morphological, stratigraphical and environmental considerations and on intraspecific and interspecific conclusions from a study on the extant species Heterodontus mexicanus Taylor & Castro-Aguirre, 1972 and Heterodontus quoyi (Frémenville, 1840) (see Migom & Hoedemakers, 2023). This led to the realization that several of the above fossil species represent, in fact, different ontogenetic stages of the same species. In this work, a specimen from the original type series of H. vincenti is formally designated as lectotype and other specimens are designated as paralectotypes. Dignathic and gynandric heterodonty is observed in H. vincenti and interspecific heterodonty with this species is demonstrated through a comparison with the above Recent species. A partial overview of the ontogenetic characteristics of the Palaeocene H. lerichei is presented. Heterodontus adneti nov. sp. is defined for specimens formerly described as Heterodontus cf. vincenti, sensu Adnet (2006). Figured specimens in Adnet (2006) are designated as holotype and paratype. Other putative Heterodontus species are identified from literature but I refrain from formally describing them as these specimens were not studied. Finally, a new set of characteristics, based amongst others on interspecific differences with the above-mentioned extant species, is suggested to identify H. vincenti and H. lerichei. Keywords: North Sea Basin, Palaeocene, Eocene, Heterodontidae, Heterodontiformes, Heterodontus vincenti, Heterodontus lerichei, Heterodontus adneti nov. sp., tooth morphology, Heterodonty, basal ornamentation, Egem, Erquelinnes, Dormaal.weiterlesen