Ehrenb. (Ehrenberg, 1838: 172, pl. 21, fig. 7)
Actinocyclus ehrenbergii Ralfs, in Pritchard (1861: 834)See VanLandingham (1967: 100) for further details of synonymy.
Hustedt (1958: 130), Priddle & Fryxell (1985: 108, fig. on p. 109), Ricard (1987: 43, figs 35–39), Hasle & Syvertsen (1997: 120)
Cells solitary, discoid. Valves slightly convex, to 50–300 µm diameter. Areolation fasciculate and with rows parallel to central row of fascicle, sometimes with hyaline areas adjacent to radial edge of fascicle. Areolae 6–8 in 10 µm, but much finer at the margin. Pseudonodulus at the junction of the valve face and mantle, distinct, operculate. A marginal labiate process is located at the end of each radial row of a fascicle.
Southern Ocean (Priddle & Fryxell, 1985); cosmopolitan (Hasle & Syvertsen, 1997); South Atlantic, c. 49–59°S 0°00’W (Hustedt, 1958).
Actinocyclus octonarius is the type species of the genus. This has rarely been recorded from polar waters.
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).