Protoperidinium steinii (Jörgensen) Balech 1974 . Rev. Mus. Argent. Cienc. Nat. “ B. Rivadavia ”, Hydrobiologia 4 (1): 63. (Figs. 65–76) .

References: Jörgensen 1899: 38; Paulsen 1908: 47, Fig. 58; Lebour 1925: 125, Pl. XXV: 125, 4 a-d; Schiller 1937: 197, Fig. 192 a–h; Kiselew 1950: 184, Fig. 304 a–g; Wood 1968: 109, Fig. 329; Taylor 1976: 159, Pl. 32: 349 a–b (as Peridinium steinii); Dodge 1982: 199, Fig. 23 C; Dodge 1985: 65; Konovalova & Selina 2010: 151, Table XX, 5 a–b; XV, 7, 8; Krakhmalnyi 2011: 207, Table 91, 1–14; Yamani & Saburova 2019: 328, Pl. 177.

Basionym: Peridinium steinii Jörgensen 1899 . Bergens Museum Aarbog 6: 38.

Synonyms: Peridinium michaelis Stein 1883, P. steinii Jörgensen 1889 .

Description: Cells pear-shaped, with concave lateral sides, slightly compressed in the cross-section. The epitheca gradually tapering into a narrow apical horn. 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 – “meta”, 2a – “penta”. The thecal surface reticulated. The cingulum median or slightly post-median, ascending (displaced by 0.5–1.0 cingular width), bears lists. The sulcus straight, shallow, with well-developed left list. The hypotheca hemispherical with two antapical spines bearing developed lists. Dimensions: 50.6–67.2 μm long and 29.4–39.6 μm wide.

Distribution: Coastal and oceanic species, widely distributed in temperate to tropical waters; cosmopolitan (Al-Yamani & Saburova 2019; Krakhmalnyi 2011). Data on the distribution of P. steinii are given from the GBIF. org (2022) (Fig. 97).