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Key to Myriopteris

1 Lower leaf surfaces with a few obscure hairs or glabrescent.
..2 Basal pinnae inequilateral (the basalmost basiscopic pinnules much larger than the basalmost acroscopic pinnules); leaf blade outline ovate to deltate, 1.5-2.5× as long as wide, 5-15 cm wide
..2 Basal pinnae equilateral, the basiscopic and acroscopic pinnules approximately equal; leaf blade outline lanceolate, 3-6× as long as wide, 1-7 cm wide.
....3 Rhizomes short-creeping, usually 4-7 mm in diameter; pinna axes (costae) green on the upper surface for most of their length; spores 32 per sporangium
....3 Rhizomes long-creeping, usually 1-3 mm in diameter; pinna axes (costae) black on the upper surface for most of their length; spores 64 per sporangium
1 Lower leaf surfaces obviously pubescent (tomentose, villous, or lanose).
......4 Petiole and rachis with hairs only (as seen at 10× magnification), the hairs segmented by prominent cell walls; margins of leaf segments more-or-less under-rolled but not modified into a scarious flap, the sori often exposed at maturity.
........5 Leaves 3-pinnate at base; ultimate segments 1-3 mm long, round (beadlike); spores 32 per sporangium
........5 Leaves 2-pinnate-pinnatifid (rarely 3-pinnate); ultimate segments 3-5 mm long, elongate (not bead-like); spores 64 per sporangium
......4 Petiole and rachis with a mixture of flattened scales (in M. tomentosa these very narrow and superficially mistakable for hairs) and hairs (as seen at 10× magnification); margins of leaf segments modified into a scarious flap (false indusium) partially to fully covering the sori.
..........6 Ultimate leaf segments scabrous (with stiff hairs) on the upper surface
..........6 Ultimate leaf segments smooth, lacking stiff hairs.
............ 7 Rachis scales ca. 0.1 (-0.3) mm wide, linear; leaf blade villous-tomentose above, appearing whitish or gray-green
............ 7 Rachis scales (0.2-) 0.4-1.0 mm wide, lanceolate; leaf blade nearly glabrous above, appearing green
............ ..8 Rhizome slender and widely creeping, leaves scattered; rachis scales ovate to lanceolate, long ciliate
............ ..8 Rhizome short and stout, leaves strongly clustered; rachis scales linear to lanceolate, lacking cilia (except maybe a few near the base)