Of uncertain derivation: from Latin os, bone + munda, cure, because the root was a remedy for rickets; or perhaps from mundae to clean, as it was used medically to clean bones. Perhaps from Osmunder, the Saxon god of war.
Rhizome: erect, massive, forming a trunk, occasionally branching, hairs and old stipe bases woven together with black, fibrous roots.
Frond: deciduous, dimorphic.
Stipe: stipules (flared leaf base), unique to the family/genus, hairy when young, vascular bundles: 1 in a U-shape where the top of the arms continue to curl.
Blade: 1-2-pinnate, rachis grooved, somewhat waxy, shedding water, reddish to light brown hairs.
Pinnae: catadromous, often jointed at the rachis, veins free, forked.
Sori: none, indusium: absent, sporangia: large, globose, tan or black when mature, spores green.
The winged stipes, the vascular bundle, and the large naked sporangium without sorus or indusium, are all unique characters.
From Bilder ur Nordens Flora (1901-1905), processing © by Dr. Gerhard Keuck.