(Figs 11, 16 K, 18 C, 20 C)
Material examined. New Zealand Antarctic Expedition TAN0402: Stn 88, Tangaroa, 14 February 2004, 72°05' 86 ''S, 172 ° 55 ' 81 ''E (Cape Adare, Ross Sea), 515 m.
Description. Monosiphonic, usually unbranched stems, up to 210 mm high, usually divided into internodes. Angle between cauline apophyses and stem 70–90 °. Perisarc strongly developed. Cauline apophyses with two axillary nematophores, each emerging through simple perisarc holes (Fig. 11 B–C).
Hydrocladia typically bifurcated (Fig. 11 A), but up to third-order branching present. First hydrocladial internode bifurcated, with two similar prongs (Fig. 11 B). Mesial inferior nematophore emerging from marked swelling at proximal third of internode (Fig. 11 D); with strongly developed nematotheca (Fig. 11 D, F). Hydrotheca elongate, roughly placed on middle of internode. Stony appearance. Abcauline wall straight, but convex at basal part. Hydrothecal aperture circular, roughly perpendicular to long axis of internode (Fig. 11 D–E, G); rim typically uneven, with slight adcauline elevation.
Gonothecae elongated with large subterminal aperture.
Remarks. The most striking feature of O. rigida is the strong stony appearance under SEM.
We have observed two axillary nematophores on every cauline apophysis examined (Fig. 11 B–C). Nevertheless, Peña Cantero et al. (1997) indicated that apophyses with a single axillary nematophore were also present. In this sense, it is worth mentioning that we have also observed a tendency to fusion of the two axillary nematophores.