Synthecium orthogonium (Busk, 1852)

Hydrorhiza tubular, reptant; stems short, to 35 mm high, lax, tubular, with two or three pairs of opposite hydrocladia, proximal stem region athecate, variable in length; diameter of stem diminishing and thickness of perisarc reducing distally; stem internodes long, nodes when present, a transverse constriction in perisarc; one or two pairs of hydrothecae about halfway to two thirds up internode; hydrocladia long and flexuous, opposite or single, given off at a slight upward angle from a strong shoulder-like apophysis.

Hydrocladial nodes marked by a pronounced narrowing of perisarc or a transverse line, hydrothecae paired, distal on internode, proximal pair displaced s lightly relative to one another; hydrothecae long, tubular, pairs may or may not be conjoined, adnate adcauline wall almost straight to faintly convex, free part bending sharply outwards and slightly forward perpendicular to hydrocladial axis for one third to half adcauline length, sometimes an internal thickening of perisarc at junction of adnate and free wall; abcauline wall slightly tumescent above base, almost straight to distal bend, then either straight or concave to margin, often an internal thickening of wall in bend. Margin parallel or almost parallel to hydrocladial axis, circular, rim strongly everted, some with up to seven marginal replications; perisarc of hydrotheca smooth.

Gonotheca on a short pedicel issuing from a hydrotheca in lower stem region, gonotheca sausage-shaped, tapering distally, body with two rows of 10 deep corrugations along one side, corrugations smoothing out behind; distal end obtuse, no orifice evident.

Colour: Live colonies mauve, white or colourless when preserved.