B.C.Booth H.J.Marchant, in Booth & Marchant (1987: 256, figs 37 (holotype), 38)
Silver et al. (1980: 212, fig. 1b), Nishida (1986: 66, pl. 1, fig. 6), Kosman et al. (1993: 124, figs 29, 30)
Cells planktonic, solitary, non-motile, spherical, 2.5–3.2 µm diameter. Cell wall comprising 8 smooth and ornamented siliceous plates: 3 shield plates, 1 ventral plate, 1 triradiate plate and 3 girdle plates; junctions between plates distinctively raised; all plates smooth. Shield plates slightly convex, with raised margins, 1.8–2 µm diameter, with or without central processes that can be rounded, spathulate or dichotomously branched. Ventral plate convex, with a narrow raised rim, with or without central irregularly branched processes 1.8–2.6 µm long. Triradiate plate with or without an undulating triradiate keel; arms of plate 1.7–1.9 µm long; margin of plate raised as a narrow smooth or irregular rim. Girdle plates with long dichotomously branched spines to 8 µm long.
Prydz Bay, East Antarctica; Terra Nova Bay; other Antarctic waters (Booth & Marchant, 1987).
Although a clear distinction can be made between subspecies ramispina and pinnatilobata, these organisms exhibit considerable structural variability. In our experience the two subspecies are the most abundant Parmales in Antarctic waters.