G.A.Fryxell, in Fryxell & Hasle (1979b: 16–19, figs 1–24)
Johansen & Fryxell (1985: 176, figs 12, 65, 66), Fryxell & Johansen (1990: 99, pl. 11.8, fig. 8), Moisan & Fryxell (1993: 293)
Cells discoid to drum-shaped; pervalvar axis 10–14 µm. Valves flat to slightly concave, circular, rounded at the margins, 16–58 µm diameter. Areolation linear to sublinear, finer on valve mantle than on valve face (Fig. d); areolae 5–6 in 10 µm in the centre, 6–7 in 10 µm near the margins, with internal pored vela (Fig. e, f). Strutted processes 1–4 in valve centre and with a single marginal ring, 2–3 in 10 µm, usually not visible by LM; each process trifultate, with a long internal extension (Fig. f). Labiate process solitary, not marginal, internally slit-like and parallel to margin (Fig. f). Valvocopula with an indistinct septum (Fig. d, e) and a complex triradiate ligula (Fig. e).
Coastal waters off Davis Station, East Antarctica; Antarctic sectors of the Pacific and Atlantic Oceans, sub-Antarctic Indian Ocean, north-western Pacific Ocean (Johansen & Fryxell, 1985).
Thalassiosira trifulta can be distinguished from T. poroseriata by having coarser valve areolation (T. poroseriata has fasciculate areolation, with areolae 11–16 in 10 µm), and from T. oestrupii by having noticeably more sparse, marginal strutted processes (T. oestrupii has 6–8 in 10 µm and eccentric valve areolation).