Marine and fresh-water species of the living dinoflagellate genus Ceratium Schrank, 1793, possess at least one hypothecal horn that is composed strictly of postcingular thecal plates, an asymmetric hypotheca with the right side shorter than the left, and a tabulation pattern that includes three plates within a concave ventral area. These ventral plates have been ignored in earlier works as contributors to the plate formula. We revise the formula from 4′, Oa, 5″, 4-5c, 5″′, 2″″ to 4′, Oa, 6″, 5-6c, 6″′, 1p, 1″″, and suggest that Ceratium is a gonyaulacinean genus. Cysts of freshwater C. hirundinella (O. F. Müller) Bergh, 1882, likewise have distinctive horns and an asymmetric hypotract but otherwise are featureless, and they do not become fossilized because they have a cellulose wall. Certain Cretaceous marine fossil dinoflagellates exhibit diagnostically ceratioid features, including postcingular horns, asymmetric hypotracts and, more rarely, the same tabulation pattern as modern Ceratium. These fossils belong to Pseudoceratium Gocht, 1957, Odontochitina Deflandre, 1935, Muderongia Cookson and Eisenack, 1958, Endoceratium Vozzhennikova, 1965, Aptea Eisenack, 1958, and Phoberocysta Millioud, 1969. These genera presumably are related to Ceratium through a pattern of iterative evolution involving selective preservation of cyst-producing taxa during the Cretaceous. Other fossil genera which we refer to as the Areoligera-Cyclonephelium complex are considered not to be ceratioid, although they are morphologically similar to those listed above in many respects. We believe that Canninginopsis denticulata Cookson and Eisenack, 1962, has a tabulation pattern that may be representative for the Areoligera-Cyclonephelium complex, and we contrast its organization with that of Ceratium.