This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol1", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
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 two-lipped marginal indusium, terminal upon the veins; ring complete, transverse, opening vertically.
Two genera, Hymenophylhun and the following, comprising some 450 or more species, abundant in the humid tropics and mainly epiphytic.