Castrac. (Castracane, 1886: 71, pl. 24, fig. 20)
Sundström (1986: 62, figs 28, 144–149), Priddle et al. (1990: 118, pl. 15.2, fig. 1, as f. sima; fig. 2, as f. silicea); Hasle & Syvertsen (1997: 150, pl. 27, tab. 33), Armand & Zielinski (2001: fig. 3L, as R. sima f. sima; fig. 13a, b, as R. sima f. silicea)
Cells usually solitary or in pairs, long, cylindrical, straight or slightly curved. Valves conical, bilaterally symmetrical; apical axis 12–39 µm (18–40 µm in f. silicea). Process bulbous, tapering abruptly to a thin-walled distal tube. Otaria large, rounded, arising on the valve apex and extending along the basal part of the process, also extending laterally. Contiguous area only distinct near the sister-cell process. Claspers conspicuous. Girdle bands forming 2 dorsiventral columns of segments, areolate; areolae 31–36 in 10 µm.
Southern Ocean, south of Australia; Admiralty Bay, Bellingshausen and Weddell Seas (Ligowski, 1993).
Rhizosolenia sima f. silicea Sundström (1986: 63, figs 29, 150–154), also recorded from southern cold-water regions, is generally regarded as the heavily silicified resting spore of f. sima. Forma silicea lacks otaria, claspers and a contiguous area, and has a robust process arising from a thickened valve apex (Hasle & Syvertsen, 1997).