Protoperidinium quadrioblongum Sarai, Yamaguchi, Kawami & Matsuoka 2013. Rev. Palaeobotany and Palynology 192: 117, Type C; Plate III, Table 4. (Figs. 5, 10) .

Basionym: Peridinium oblongum var. symmetricum Dangeard 1927: 7–10, Fig. 5, Figs. 1a–1c.

Description: Cells pentagonal, with an apical and two large antapical horns. The apical pore structure of Bʹ– type (Toriumi & Dodge 1993). The epithecal plate pattern comprises seven precingular plates (7ʹʹ), four apical plates (4ʹ), three anterior intercalary plates (3a), 1ʹ plate – “ortho”, 2a – “quadra”. The thecal surface finely reticulated, with scattered pores. The cingulum median, descending (displaced by 1.0 cingular width). The sulcus straight, broad, bears lists. Dimensions: 70–106 μm long and 50–65 μm wide.

Nomenclatural note: Sarai et al. (2013) proposed to split up Protoperidinium oblongum into three separate taxa of species rank: a) Protoperidinium quadrioblongum Sarai et al. 2013 (2a – “quadra” type), which incorporates Peridinium oblongum var. symmetricum Dangeard 1927: 7–10, Fig. 5 and Peridinium oblongum (Aurivillius) in Lebour 1925: 121, pl XXIV, Figs. 1a–1c; b) Protoperidinium paraoblongum Sarai et al. 2013 (2a – “penta”) incorporating Peridinium oblongum var. inaequale Dangeard 1927, 7–10, Fig. 6; c) Protoperidinium latidorsale (Balech) Balech 1974 (1a – “hexa”) – Peridinium oblongum var. latidorsale Dangeard 1927, 7–10, Fig. 7.

Unfortunately, in publications pertaining to the Black Sea, only the species name “ Protoperidinium oblongum ” was mentioned, without specifying the variety, and presenting neither drawings nor photographs. In specimens of P. oblongum (Aurivillius) Parke & Dodge found by us, the first apical plate (1ʹ) is “ortho”, and 2a – “quadra”. Thus, taking into account the study of Sarai et al. (2013), the specimens should be assigned to P. quadrioblongum Sarai et al. 2013 . We did not observe other types of epitheca morphology in this species in the Black Sea.

Protoperidinium quadrioblongum ( P. oblongum) is close to P. oceanicum, which is primarily distinguished by larger size (120–300 μm long and 60–150 μm wide) and relatively longer apical and antapical horns.

Distribution ( P. oblongum): Neritic, oceanic; cosmopolitan in cold temperate to tropical waters (Steidinger & Tangen 1997). Data on the distribution of P. oblongum are given from the GBIF.org (2022) (Fig. 95).