Thallus consisting of root-like holdfast, short stipe and blade. Blade long-belt shaped, up to one meter long, 10-20 cm broad, with margin undulate and overlapping, thick at the middle and thin at the margin. A short and small stipe and holdfast at the base of the blade. Holdfast sturdy (presenting haptera) with which the algae is fixed to rocky substratum. Colour: thick dark green; blade surface brown, occasionally glaucescent.