Stems compact, ascending, stout, 5--10 mm diam.; scales uniformly reddish brown, linear-subulate, 0.1--0.3 mm wide, thin, margins entire to denticulate. Leaves somewhat dimorphic, sterile leaves shorter than fertile leaves, clustered on stem, 8--25 cm; croziers villous. Petiole reddish purple to dark brown, lustrous, rounded adaxially, without prominent articulation lines. Blade elongate-deltate to lanceolate, 2-pinnate proximally, 3--6 cm wide; rachis purple or brown throughout, straight, rounded adaxially, sparsely villous with long, divergent hairs. Pinnae ascending or perpendicular to rachis, not decurrent on rachis or obscurely so, usually with 3--7 ultimate segments; costae straight, 2--30 mm, usually shorter than ultimate segments. Ultimate segments oblong-lanceolate, 7--30 mm, leathery, sparsely villous abaxially near midrib; margins usually recurved on fertile segments, covering less than 1/2 abaxial surface, borders whitish, crenulate; apex obtuse to slightly mucronate. Veins of ultimate segments obscure. Sporangia long-stalked, containing 32 spores, not intermixed with farina-producing glands.

Sporulating summer--fall. Calcareous cliffs and ledges, usually on limestone; 100--1500 m; Alta., B.C., Sask.; Mo., S.Dak., Wyo.

Pellaea gastonyi is an apogamous tetraploid that has originated through repeated hybridization between P . atropurpurea and P . glabella . Isozyme studies (G. J. Gastony 1988) indicate that P . glabella subsp. missouriensis was the diploid parent of plants found in Missouri, whereas diploid P . glabella subsp. occidentalis was involved in the origin of P . gastonyi populations occurring in western North America. Pellaea gastonyi is most often confused with P . atropurpurea , from which it differs in having sparsely villous rachises, smaller ultimate segments, and spores averaging more than 62 µm in diameter.