Polyplumaria cornuta (Bale, 1884)

Hydrorhiza a tangle of thin colourless stolons embedded in shell grit.

Stems solitary, to 280 mm long, slender and flexuous, a proximal region short to long and sometimes lightly fascicled, stem thereafter hydrocladiate, monosiphonic; distal half to third of stem branched in verticels of three, branches short, reflexed, hydrocladiate. Perisarc of stem and branches thick, smooth.

Hydrocladia on branches alternate, on an indistinct apophysis, hydrocladial internodes thecate, short, nodes strongly oblique, deep, three strong septa passing downwards from base of hydrotheca almost through internode; two partial septa, one below mesial nematotheca and between lateral nematotheca and node. Apophyses of some branches with a spinous accessory hydrocladium with several intrathecal septa, one or two nematothecae near apex and one hydrotheca with twin laterals but no mesial nematotheca.

Hydrotheca adnate to internode, adcauline wall obscured by three humps associated with intranodal septa, abcauline wall weakly sinuous with a small peak behind margin; margin strongly sinuate, cut back to meet internode.

Nematothecae all bithalamic, movable, mesial inferior on a small prominence of internode, an infilling of perisarc at base, nematotheca standing almost erect, basal chamber long, slender, cup shallow, foreshortened on adcauline side to almost triangular; basal chamber of lateral moderately long, inserted at base of hydrothecal margin, cup rather shallow, broad, a little excavated on adcauline side. Cauline nematothecae similar to mesial; three on internode, one at base of apophysis and two in axil.

Gonotheca inserted without pedicel in axil of apophysis, small, almost triangular, widening distally, no operculum, perisarc very thin.

Colour: In life, very pale yellow-brown, lower stems darker brown.