This section is from the book "Handbook Of Hardy Trees, Shrubs, And Herbaceous Plants", by W. Botting Hemsley. Also available from Amazon: Handbook of hardy trees, shrubs, and herbaceous plants.
Rootstock creeping. Fronds circinnate in vernation, very delicate and almost transparent, with reticulate veins. Involucre bivalved or bilabiate. Spore-cases sessile, on a clavate or filiform receptacle surrounded with a complete oblique or transverse ring.