Halopteris opposita (Mulder & Trebilcock, 1911)

Stems to 12 cm long, with 2-3 indistinct proximal annulations, followed by a long athecate internode with 2-3 nematothecae, then alternate thecate and athecate internodes of approx. the same length, 0.27-0.39 mm; athecate internodes with a proximal transverse joint and strongly oblique distal hinge-joint, and 2-3 nematothecae; if 2, they are one above the other; the third, if present, is beside the distal nematotheca.

Hydrocladia opposite, arising behind the cauline hydrothecae in middle of the internode, hydrocladia beginning with 2 short athecate internodes, the first internode the shorter, 0.05-0.07 mm long, the second 0.07-0.08 mm long, with a transverse proximal, and an oblique distal joint, and occasionally, 1 nematotheca. Thecate hydrocladial internodes similar to stem internode.

Hydrothecae campanulate, 2-3 on hydrocladium, 0.15-0.18 mm deep, set at 45° to hydrocladial axis. Margin 0.19-0.21 mm diameter, slightly sinuated, rim everted, with a peak on the abcauline side. Abcauline wall 0.12-0.15 mm long, in some hydrothecae a small transverse fold near base.

Nematothecae 0.05-0.07 mm long, 2 suprathecal on stem internodes, 4 on thecate hydrocladial internodes-l median, stout at base, distal cup cut away on adcauline side, closely adpressed to internode; 2 lateral suprathecal, with slender pedicels, on an apophysis of the internode, extending above hydrotheca, and 1 small nematotheca between laterals at base of hydrotheca.

Hydranth with 10-16 tentacles, connected to internode by a small orifice in the upcurve of the abcauline wall.