by Nathaniel Lord Britton & Addison Brown, An Illustrated Flora of the Northern United States… (1913)
“ Membranaceous, mostly tropical small ferns, with slender often filiform creeping or rarely suberect rootstocks, the leaves usually much divided, the leaf-tissue pellucid, usually of a single layer of cells. Sporanges sessile upon a filiform, usually elongate receptacle, within an urn-shaped or tubular truncate or twolipped marginal indusium, terminal upon the veins ”
