Mangin (1917: 9, figs 5–7)
VanLandingham (1968: 709), Evensen & Hasle (1975: 158, figs 15–22), Hasle & Syvertsen (1997: 199, pl. 40)
Cells in straight chains, heterovalvate, rectangular in girdle view, with upper face rounded; pervalvar axis 13–20 µm. Valves elliptical, 12–30 µm diameter; upper valve rounded, with setae arising from near the centre; lower valve flat, with setae arising from inside valve margin; small central spine present. Apertures between cells small, ±trapezoid. Setae long, thin at point of emergence from valve, becoming broader distally, finely spinose for much of their length. All setae curving back towards lower end of chain. Chloroplasts small, spherical.
Bransfield Strait (Kopczyńska & Ligowski, 1985); Ross Sea (Watanabe, 1982).
Hasle & Syvertsen (1997) reported this as a “cosmopolitan species, common in temperate and northern cold waters but absent from southern cold waters”. It is included here because of the few records from more southerly locations. Its morphology and size are similar to C. peruvianus, cells of which are also heterovalvate and with downward-curving, spinose setae. However, C. peruvianus has a pronounced constriction between the valve mantles and a narrow girdle in girdle view (this area is straight in C. concavicornis). Moreover, the setae arise as a distinct, vault-like structure from the centre of the upper valve.