(O.G.Kozlova) G.A.Fryxell & Hasle (1983: 54–57, figs 1–14)
Porosira dichotomica O.G.Kozlova (1962: 8, fig. 2; fig. 3, no. 3)
Kozlova (1967: 56, figs 2, 11, as Porosira dichotomica), Johansen & Fryxell (1985: 161, figs 21–23, 33–35), Fryxell & Johansen (1990: 99, pl. 11.9, fig. 7–9), Hasle & Syvertsen (1997: 63)
Cells apparently solitary, with chitinous strands only observed from the marginal strutted processes. Valves flat to slightly depressed (Fig. d, f), 12–26 µm diameter; valve face weakly silicified, difficult to observe by LM (Fig. a). Areolae in a distinct tangential (Fig. d) or radial (Fig. e) arrangement on valve face; radial areolae 25–30 in 10 µm; one elongate areola between margin and ring of strutted processes (Fig. f). Areolae with thickened walls, loculate, opening externally via a simple foramen and internally by an evenly pored velum (e.g. marginal areolation of Fig. b). Single marginal ring of strutted processes, 3–7 in 10 µm. Central processes 1–3 (–6) (Fig. b–e). Strutted processes with short external extensions (Fig. d) supported by thickening from adjacent areola walls; internal opening operculate, almost sessile and orientated along the radius (Fig. b). Labiate process solitary, just inside marginal ring of strutted processes and between 2 strutted processes (Fig. b, d, f). Girdle with an areolate valvocopula, a largely unstructured and narrow copula, and 2 or 3 unstructured pleurae (Fig. f). For a complete description, see Fryxell & Hasle (1983).
ADBEX (Southern Ocean, south of Australia); type locality, Indian Ocean sector of the Antarctic (Kozlova, 1967); Pacific and Atlantic sectors of the Southern Ocean (Fryxell & Hasle, 1983); probably with a circum-Antarctic distribution.
A distinctive feature of T. dichotomica is the concentric pattern of thickened areolae walls formed on the valve face (Fig. e). Material examined in the current study has a density of 25 areolae in 10 µm; a higher density (25–30 in 10 µm) was reported by Johansen & Fryxell (1985). Our material also has 6–7 strutted processes in 10 µm and a variable number (?0–) 1–3 of central processes (Fig. b–e), whereas Johansen & Fryxell (1985) reported 3–7 strutted processes in 10 µm and 1–6 central processes.