Cartilaginous, usually markedly compressed, blackish-purple bleaching to pale yellow fronds, usually 20-80 mm long, forming extensive, closely interwoven turfs from entangled stolonous holfasts. Very variable in size and form. Main axis usually simple, branching alternate distichous, repeatedly pinnate. Ultimate ramuli short, blunt.

Source: http://www.seaweed.ie/descriptions/Osmundea_pinnatifida.php