Comber (1896: 491, pl. 11)
Thalassiosira antarctica G.Karst. (Karsten, 1905: 73, pl. 2, figs 2, 3), nom. inval.Thalassiosira fallax Meunier (1910: 268, pl. 30, figs 1–4) (see Syvertsen 1979: 59)Thalassiosira antarctica var. borealis G.A.Fryxell, Doucette & G.F.Hubb. (Fryxell et al., 1981: 323, figs 21–24, 28–33)See VanLandingham (1978: 3992) for full synonymy.
Hendey (1937: 237), Manguin (1954: 22), Hasle & Heimdal (1968: 357, figs 1–23), Fryxell et al. (1981: 322–332, figs 1–5, 11–13, 17–20, 26, 27, 35A, B), Semina & Makarova (1983: 613, pl. 2, figs 5, 6; pl. 4, figs 1–6; pl. 5, figs 1–6), Johansen & Fryxell (1985: 158, figs 15–17, 37–39), Fryxell & Johansen (1990: 99, pl. 11.9, figs 1–3), Hasle & Syvertsen (1990d: 289, figs 25–27), Moisan & Fryxell (1993: 293), Hasle & Syvertsen (1997: 66, pl. 8, tab. 10)
Cells chain-forming, held together by chitinous strands emerging from central cluster of strutted processes (Fig. a, d); distance between cells usually 2–3 times as great as pervalvar dimensions of cells. Larger cells less strongly silicified. Valves circular, 14–56 µm diameter, becoming markedly convex with reduction in size. Areolation radial, bifurcate to fasciculate; areolae 17–20 in 10 µm, loculate, opening externally via a simple foramen, internally via an evenly pored velum (Fig. b–d). Strutted processes 2–several, 4- or 5-sided, operculate (Fig. c), with small external extensions not visible by LM, 8–16 in 10 µm on mantle, 4–14 in 10 µm in the centre. Labiate process large, solitary, sessile, located in outer ring in valves with 1 marginal ring (Fig. b, d), but located between rings in those valves with 2 or 3 marginal rings of strutted processes. Internal opening of labiate process aligned with radius of valve (Fig. b, d); external opening elongated and similarly orientated. Cingulum with an areolate valvocopula; copula with a line of pores proximal to valvocopula and some additional pores on the ligula. Pleurae apparently unstructured, 3 or 4 and spiralling to the right. For a complete description, see Fryxell et al. (1981) and Johansen & Fryxell (1985).
Type locality, South Atlantic, in the area of the South Shetland Is.; Southern Ocean and sub-Antarctic (Fryxell et al., 1981); Heard I. (Manguin, 1954); Weddell-Scotia Confluence (Garrison et al., 1987); Fryxell et al. (1981) also described an Arctic form of this bipolar species.
Large cells observed in the current study had only one row of marginal processes. The smaller cells, which also resemble T. scotia (see below), have 2 rings of marginal strutted processes with the labiate process formed between the rings. None of the cells observed showed any evidence of occluded processes described for the typical T. antarctica and T. scotia by Johansen & Fryxell (1985). The smaller cells are similar to T. scotia in the formation of resting, heterovalvate cells. Hasle & Syvertsen (1997) and Fryxell et al. (1981) outlined the reasons for the earlier segregation of T. antarctica var. borealis.