Further observations on a hypothecal pore in the genus Protoperidinium Bergh (Dinoflagellata)

The data on 21 Protoperidinium species (including 8 species assigned to the genus Peridinium, which supposedly also belong to the genus Protoperidinium) with a hypothecal pore on the first postcingular plate are summarized. The hypothecal pore is shown to be present not only in the species with the first apical plate of the para-type, but also in some species with the meta-type. It is confirmed that the pore is associated with the hexa second intercalary plate. The pore can be present in closely related species and is reliable diagnostic feature to differentiate Protoperidinium species. At least five groups of morphologically similar and supposedly related taxa can be distinguished: (1) Protoperidinium affine, P. pallidum and P. pellucidum; (2) P. curvipes and P. variegatum; (3) P. diabolum and P. dodgei; (4) P. cruciferum, P. cf. cruciferum, Protoperidinium sp. 1 (meta-hexa) and Peridinium acutum; (5) P. ovum and Protoperidinium sp. 2 (para-penta).