The name Thelypteris thelypterioides (Michaux) Holub has been applied to T . palustris , but A. F. Tryon et al. (1980) have shown that this was a result of an incorrect typification by C. V. Morton (1967). Thelypteris palustris var. palustris occurs in Eurasia.
Rhizome with a few appressed, ovate scales, otherwise glabrous; lvs subdimorphic; petioles black at the base, glabrous, scaleless, the sterile to 3.5 dm, shorter or longer than the blades, the fertile to 7 dm, longer than the blades; blade lanceolate or lance-oblong, to 4 נ2 dm, pinnate-pinnatifid, acuminate, only slightly narrowed at the base; pinnae lance-linear, subsessile, the sterile to 2 cm wide, with obtuse, entire, broadly oval segments to 10 נ5.5 mm, the fertile segments oblong, to 4 mm wide, with incurved margins; costae scarcely scaly, these and the costules hairy; veins 6-8 pairs per segment, those of the sterile blades once forked, those of the fertile blades simple or the basal ones mostly forked; indusia ciliate or rarely glabrous; 2n=70. Marshes and bog-margins; widespread in the N. Hemisphere, and throughout our range. (Aspidium thelypteris; Dryopteris thelypteris) Our plants are var. pubescens (Lawson) Fernald.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
From Flora of the Indiana Dunes (1930) - Donald C. Peattie
T. palustris (L.) Schott. Low, the fronds pinnate, in outline lanceolate; pinnae deeply pinnatifid, horizontal or slightly recurved, with oblong lobes; margins strongly revolute, making the pinnules appear acute at the tip; fruit dots numerous, soon merging. (T. Thelypteris House; Aspidium Thelypteris Sw.; Dryopteris Thelypteris Gray.)—A pale green, fragrant fern, abundant on marshes, in swales, dune meadows, and moist prairies. August.