A branched filamentous organism from the Lower Carboniferous of Scotland, described by Kidston under the name of Bythotrephis worstoniensis, shows some remains of cellular structure, and may probably be a true Alga, resembling some of the filamentous Florideae in habit.
The conical papillae are small and close set, though longer and more filamentous on the intermolar portion.
Those that bear the antheridia are the smaller, and are either filamentous, or flattened, and irregularly lobed.
Its surface is clothed with filamentous or scaly hairs (paleae), which protect the growing point; and adventitious roots spring from it.
The dorsiventrality of the prothallus has been shown to depend mainly on the illumination, the filamentous form being retained in feeble light; a similar result is obtained when the prothalli are cultivated in water.