Leaves monomorphic, dying back in winter, 25--90 × 15--30 cm. Petiole 1/3 length of leaf, scaly at least at base; scales scattered, brown, sometimes with darker patch at base. Blade light green, deltate-ovate, 3-pinnate-pinnatifid, herbaceous, not glandular. Pinnae in plane of blade, lanceolate; basal pinnae deltate to broadly lanceolate, not reduced, basal pinnules equal to adjacent pinnules, basal basiscopic pinnule much longer and 2 times width of basal acroscopic pinnule (only in this species); pinnule margins toothed, teeth spine-tipped. Sori midway between midvein and margin of segments. Indusia usually lacking glands. 2 n = 164. Cool, moist woods at increasing elevation southward; frequently only at summits of mountains; 0--1500 m; N.B., Nfld., N.S., P.E.I., Que.; Conn., Maine, Md., Mass., N.H., N.Y., N.C., R.I., Tenn., Vt., Va., W.Va.
Rhizome horizontal, short-creeping; lvs deciduous, rather widely ascending-spreading, the petiole commonly about as long as the blade, beset with light brown, concolorous scales; blade mostly 3-5 נ2-3 dm, triangular to pentagonal, tripinnate-pinnatifid, with ca 15-20 pairs of pinnae; first two pinnules of the basal pinna offset by 4-12 mm, the basiscopic one evidently wider than the acroscopic one and (2-)3-5 times as long; ultimate segments finely spinulose-toothed; sori midway between the midvein and the margins; indusium sometimes with some stipitate glands, the lf otherwise glandless; rhizome horizontal, short-creeping; 2n=164, usually thought to be an alloploid of spp. 8 and 11 but hardly to be distinguished morphologically from the latter. Moist woods; Lab. and Nf. to s. Que., w. Mass., N.Y., and Pa., and s. in the mts. to N.C. and Tenn. (D. spinulosa var. americana; D. austriaca, misapplied)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.