Petalonia zosterifolia
(Reinke) Kuntze, 1898

Thallus narrow-ribbon-like, flattened, simple, dark brown. Central part composed of a few layers of colourless cells, cortex single-layered. Plurilocular sporangia uniseriate, crowded, covering large areas of the blade. Loose speciments were once found on the shore. Boreal species.