Janisch (1878), in Schmidt et al. (1874–1959: pl. 57, fig. 31)
Coscinodiscus curvatulus var. subocellatus Grunow (1884: 83, pl. D, fig. 15)Actinocyclus subocellatus (Grunow) Rattray (1890a: 488; 1890b: 145)
Simonsen (1974: 20), Priddle & Fryxell (1985: 106, figs A, B), Hasle & Syvertsen (1997: 121, pl. 19)
Cells solitary, discoid, 13–160 µm diameter. Areolae fasciculate, in slightly but distinctly curved rows; areolae 8–18 in 10 µm, decreasing in size towards the margin, with a distict marginal band formed of small areolae. Marginal processes 12–18 in 10 µm, each located at the end of a row of areolae; processes expanded on internal valve face (Fig. d). Pseudonodulus near the margin (Fig. a), non-perforate.
Coastal waters near Davis Station, East Antarctica; type locality, Zemlya Frantsa-Iosifa (Franz Josef Land), Arctic Ocean (Grunow, 1884); Kerguelen Is. (Grunow, 1884); South Atlantic, 46°32’S 0°02’W–49°00’S 0°07’W (Hustedt, 1958).
The fasciculate areolae in slightly curved rows are an especially distinctive feature of this species. Valve diameter of A. curvatulus seems to be rather variable, e.g. 36–57 µm (Hustedt, 1958) and 124–160 µm (Mann, 1937). Cells collected near Davis Station were 25–40 µm in diameter, with fasciculate areolae 8 in 10 µm in the centre, c. 15 in 10 µm and decreasing in diameter towards the margin, and labiate processes 1–2 in 10 µm in a marginal ring.
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).