Macrorhynchia
Diagnostic description:
Colony erect, hydrocauli branched or unbranched, polysiphonic, arising from a creeping hydrorhiza or anchoring filaments; hydrocladia pinnate, unbranched, arising alternately from apophysis on axial tube of stem and branches, hydrothecae borne only on hydrocladia, more or less sac-shaped, hydrothecal margin dentate, intrathecal septum present; cauline internodes with triangular nematothecae, each hydrotheca with a pair of lateral nematothecae and a partly adnate median nematotheca; gonothecae on unbranched phylactocarps of modified hydrocladia, single or in aggregated pseudomorphs; sporosacs fixed.