Analysis of the thecal plate pattern in Katodinium rotundatum (Lohmann) Loeblich has revealed that the tabulation is identical to that of Heterocapsa Stein. Other similarities between Katodinium rotundatum and Heterocapsa include the presence of triangular scales on the cell surface, the pyrenoid structure and the structure of the flagellar apparatus. Katodinium rotundatum is transferred to Heterocapsa as this name has priority. It is indicated that scale morphology is superior to thecal plate pattern for identification of Heterocapsa species and should be included in future diagnoses of them. The organism described as Heterocapsa quadridentata Stein is transferred to Peridinium, as Peridinium quadridentata (Stein) comb. nov.