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Roots not proliferous. Stems short-creeping to erect, often branched; scales very dark brown throughout, narrowly deltate, 1--3 × 0.1--0.25 mm, margins with widely spaced teeth. Leaves monomorphic. Petiole reddish brown proximally, green distally, dull, 1--9 cm, (1/2--)1--2 times length of blade; indument of dark brown, narrowly deltate scales proximally grading into multicellular hairs. Blade deltate-ovate to obovate or oblanceolate, 1--2(--3)-pinnate to 2-pinnate-pinnatifid, 2--6 × 1--4 cm, somewhat thick, glabrous; base obtuse; apex acute to rounded, not rooting. Rachis green, dull, glabrous except for very sparse, minute hairs. Pinnae in 2--4 pairs, deltate-ovate to obdeltate; proximal (largest) pinnae 7--30 × 5--20 mm; base truncate to acute; margins finely (sometimes coarsely) incised; apex rounded to acute. Veins free, evident. Sori as many as 30 or more per pinna, usually 1--5 per segment, on both basiscopic and acroscopic sides. Spores 64 per sporangium. 2 n = 144.
Rhizome 1.5–2 mm thick, the lanceolate scales 1.5–4 mm; lvs numerous, spreading, 4–17 cm, the petiole to 7 cm, green blade ovate or rhombic, firm, bipinnate or bipinnate-pinnatifid below, merely pinnate above, the rachis green; pinnae alternate, ca 6 pairs, long-petiolulate (petiolule 2–5 mm), the basal ones with 3–6 flabellate or rhombic, finely toothed and sometimes deeply lobed pinnules to 12 mm; 2n=144. Rock-crevices and ledges, mostly on limestone; interruptedly circumboreal, in Amer. from Mass. and Vt. to Mich., s. to Ala., Ill., Mo., and Ark. (A. cryptolepis, the Amer. pls.)
A fern.
