(Ehrenb.) Ehrenb. (Ehrenberg, 1843: 400, pl. 1.1, fig. 27; pl. 1.3, fig. 21)
Actinocyclus undulatus Kütz. (Kützing, 1844: 132)Actinoptychus undulatus Bailey ex Ralfs, in Pritchard (1861: 839, pl. 5, fig. 88)Actinoptychus undulatus Bailey ex Hustedt (1927–30: 475), nom. illeg.
Actinocyclus senarius Ehrenb. (Ehrenberg, 1838: 172, pl. 21, fig. 6)
Bailey (1842: 94, fig. 11, as Actinoptychus sp.), Hendey (1937: 271), Priddle & Fryxell (1985: 110, figs A–C), Round et al. (1990: 200, fig. A), Hasle & Syvertsen (1997: 141, pl. 22)
Cells solitary, discoid. Valves divided into 6 sectors, alternately raised and depressed, 20–150 µm in diameter; central area plain and ±hexagonal; remainder of valve with coarse areolation; areolae 4–7 in 10 µm. Each of the raised sectors has a marginal labiate process with an external tube. Choroplasts numerous, large (Hasle & Syvertsen, 1997).
Southern Ocean (Priddle & Fryxell, 1985); South Atlantic, Southern Ocean off Enderby Land, South Georgia, Drake Passage (Hendey, 1937).
Primary valves were formerly described as the genus Debya Pant. (Round et al., 1990). Hendey (1937) noted that Actinoptychus senarius was often attributed as “Actinoptychus undulatus Bailey”, but that there were no grounds for this, Bailey’s original figure having been unnamed.
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).