Watkins, in Watkins & Fryxell (1986: 310, figs 17–23)
Fryxell (1990: 112, tab. 14.1, pl. 14.1, fig. 3), Hasle & Syvertsen (1997: 122, tab. 26)
Cells solitary, cylindrical to discoid. Valves flat, with rounded edges, 27–36 µm diameter. Areolation variable; areolae in curved or incomplete rows, 13–15 in 10 µm, abruptly changing to 24–29 in 10 µm at the margin, loculate, with a foramen on the inside, bordered by a thickened rim. Hyaline margin absent. Marginal labiate processes 8–12, slightly expanded, 10–12 µm apart. Chloroplasts numerous, discoid.
Southern Ocean, 62°05.4’S 39°05.6’W, AMERIEZ 83, Station 16 (Watkins & Fryxell, 1986); Antarctic ice-edge (Fryxell, 1990).
The rare Actinocyclus spiritus is morphologically most similar to A. actinochilus, the latter having coarser valve areolation (5–11 in 10 µm).
Species of Actinocyclus have the following diagnostic features: radial, usually fasciculate areolation; a marginal ring of labiate processes, laterally expanded internally; a marginal pseudonodulus (sometimes difficult to distinguish); and radial, hyaline bands (see Villareal & Fryxell, 1983 and Watkins & Fryxell, 1986 for further details).