Previous accounts of the dentition of the Carboniferous dipnoan Uronemus have stressed the significance of the scattered small denticles. These, together with the marginal teeth and ridges, have been interpreted as primitive characters of the dipnoan dentition shared with three other genera: the Devonian Uranlophus and Griphognathus and the Carboniferous to Permian Conchopoma. Genera with tooth plates have been considered to be a monophyletic group in which tooth plates are a derived character; Uronemus has been excluded from this group in all previous investigations dealing with the significance of the dentition for determining relationships among dipnoans. The macromorphology of the dentition of Uronemus has been re-examined and correlated with the histology of all the dental tissues. Optical study of thin sections and scanning electron microscope study of the adjacent cut surfaces has shown that the hard, wear-resistant dentine of the teeth and ridges is petrodentine. The arrangement, growth, wear and histology of the dental tissues have been compared with those of denticulated and tooth-plated genera. The arrangement of new teeth relative to the tooth ridge, the pattern of wear along the ridge, and the type of dentine and its growth indicate that the dentition of Uronemus is best interpreted as a tooth plate with one long lingual tooth ridge and reduced lateral tooth rows. Therefore the marginal tooth ridges are not considered to be homologous with those of denticulate dipnoans such as Uranolophus. The presence of petrodentine, a tissue type only found in forms with tooth plates, is consistent with the view that the dentition is derived by modification of a radiate tooth plate. The denticles covering restricted regions of the palate and lower jaw are considered to have been a secondary acquisition. The suggestion that Conchopoma is a close relative of Uronemus is not accepted, and possible new relationships have been proposed. New data on Scaumenacia and Phaneropleuron, two other genera previously compared with Uronemus, are presented. Rhinodipterus, a form with elongate lingual ridges, is also discussed. Phaneropleuron is shown to have radiate tooth plates and not a marginal row of conical teeth as previously described. It is proposed that the tooth plate of Uronemus is derived from a dipterid type of plate. A discussion of some of the other factors involved in determining the relationships of the genus is given.(ABSTRACT TRUNCATED AT 400 WORDS)