Protoperidinium achromaticum (Levander) Balech 1974 . Rev. Mus. Argent. Cienc. nat. “ B. Rivadavia ”, Hidrobiol. 4 (1): 56. (Figs. 1–2) .

References: Lebour 1925: 114, Pl. XXII, Fig. 1 a–g; Abé 1927: 412, Fig. 31; Schiller 1937: 229, Fig. 225 a–h; Kiselew 1950: 195, Fig. 330 a–e; Taylor 1976: 138, Pl. 33: 371 a–b (as Peridinium achromaticum); Dodge 1982: 183, Fig. 21 B; Krakhmalny 2011: 190, Table 75, Figs. 8–15; Al-Yamani & Saburova 2019: 294, Pl. 161.

Basionym: Peridinium achromaticum Levander 1902 . Medd. Soc. Profauna Fl. Fennica: 49, Figs. 1, 2.

Description: Cells rhomboid. The epithecal plate pattern comprises seven precingular plates (7ʹʹ), four apical plates (4ʹ), three anterior intercalary plates (3a), and “ortho”–”hexa” combination of the 1ʹ and 2a plates. The thecal surface smooth, with scattered pores. The cingulum median, deeply excavated, almost circular. The sulcus slightly broadens towards the antapical portion of the cell, with two sulcal lists protruding posteriorly giving the appearance of small antapical spines in ventral view. Dimensions: 27–48 μm long and 23–40 μm wide.

Distribution: Neritic species. Reported from cold temperate to tropical low saline waters (Al-Yamani & Saburova 2019). Data on the distribution of P. achromaticum are given from the GBIF.org (2022) (Fig. 77).