Castrac. (Castracane, 1886: 78, text fig.)
Hendey (1937: 295, pl. 13, fig. 7), Hustedt (1958: 135), VanLand-ingham (1968: 712), Priddle & Fryxell (1985: 30, as C. criophilum), Everitt & Thomas (1986: 5, fig. 2A), Fryxell (1989: 10, figs 33, 34), Hasle & Syvertsen (1997: 193, pl. 38)
Cells solitary or chain-forming, ±rectangular in girdle view, heterovalvate; pervalvar axis 15–55 µm. Valves almost circular; apical axis 16–50 µm diameter; upper valve convex; lower valve flat. Setae long, curved back towards lower end of chain, spinose throughout their length; setae of upper valve arising near the centre as part of the valve /surface (Fig. b), those of lower valve arising from near valve margin; a characteristic notch between setae and valve face on the lower valve. Chloroplasts numerous, small, spherical, sometimes distributed into the setae.
Inshore waters of Davis Station, East Antarctica (Everitt & Thomas, 1986); South Georgia, Weddell Sea, Bellingshausen Sea, Ross Sea (Hendey, 1937); Lützow-Holm Bay (Tanimura et al., 1990); widespread in Antarctic and South Atlantic especially from 61°32’S 0°00’ to 53°43’S 0°20’W (Hustedt, 1958); Weddell-Scotia Sea (Garrison et al., 1987).
This is a characteristic Antarctic diatom (Hendey, 1937). Although resting spores are not known, lipid-rich, relatively dormant cells may be the ‘winter’ forms (Fryxell, 1989).