Stem scales brown to nearly black. Leaves 4--15 cm. Petiole dark brown, 0.75--1.5 mm diam. Blade ovate-lanceolate, 2--3-pinnate proximally, leathery, abaxially and adaxially glabrous; rachis rounded to slightly flattened adaxially. Pinna costae straight or nearly so, branches not arising from prominent angles. Ultimate segments not articulate, dark color of stalks continuing into segment bases abaxially; segment margins plane to slightly recurved, not concealing sporangia; veins obscure adaxially. Sporangia submarginal, borne on distal 1/2 of secondary veins, containing 64 spores. 2 n = 54, 108. Sporulating spring--fall. Calcareous cliffs and ledges; 600--1900 m; Ariz., Calif., Nev., Utah; Mexico in Sonora. Argyrochosma jonesii includes two sexually reproducing cytotypes. The diploid is known from a few localities in the Sonoran and Mojave Deserts; the tetraploid is found throughout the Mojave Desert and cismontane southern California. Although subtle morphologic differences exist between these cytotypes, they are distinguished primarily by characteristics known to correlate with increases in ploidy level (such as spore size and the length of stomatal guard cells). Further investigation is necessary to determine whether the tetraploid arose through autopolyploidy or hybridization between cryptic species.