To clarify the systematic positions of the important gonyaulacacean generaOperculodiniumWall, 1967 emend. Matsuoka et al., 1997 andProtoceratiumBergh, 1881, we present in detail the tabulation of the Oligocene–Pleistocene, thermophilic, cyst-defined speciesOperculodinium bahamenseHeadinHead and Westphal, 1999 emend., and the extant, cosmopolitan, theca-defined speciesProtoceratium reticulatum(Claparède and Lachmann, 1859) Bütschli, 1885. Both species have a sexiform hyposomal tabulation, and L-type (Protoceratium reticulatum) or modified L-type (Operculodinium bahamense) ventral organization.Protoceratium reticulatumhas dextral torsion of the hypotheca, requiring assignation of the genus to the subfamily Cribroperidinioideae Fensome et al., 1993, whereasOperculodinium bahamensehas neutral torsion requiring assignation to the subfamily Leptodinioideae Fensome et al., 1993. The stratigraphic range of this subfamily is now extended upwards to the lower Pleistocene. Paradoxically,Protoceratium reticulatumproduces a cyst whose morphology is circumscribed by the cyst-defined genusOperculodinium, either implying polyphyletic origins for this genus or that combinations of ventral organization and torsion used to subdivide the family Gonyaulacaceae cannot always be applied rigidly. In detail,Operculodinium bahamenseis shown to have an unusual ventral tabulation in which the first apical plate contacts the apical pore complex but not the sulcus. The new term “episert” is proposed to describe this plate relationship, which appears to have evolved independently in several lineages of the suborder Gonyaulacineae.