Tenuis means slender.
Rhizome: short-creeping, covered with old stipe bases, scales tan to light brown, lanceolate.
Frond: 40 cm high by 6 cm wide, deciduous, monomorphic, blade/stipe ratio: 3:2.
Stipe: stipes --3 to 8-- clustered at stem apex, grooved, dark brown at base, straw-colored above, sparsely scaly at the base, vascular bundles: 2, round or oblong.
Blade: 2-pinnate-pinnatifid, ovate-lanceolate, herbaceous, glabrous.
Pinnae: lanceolate, widest just below the middle, angled upward from the rachis and curving towards the tip, ± opposite; costae grooves above continuous from rachis to costae; margins crenulate or with rounded teeth; veins free, simple or forked, directed into teeth and notches.
Sori: round, in 1 row between midrib and margin, indusium: ovate to lanceolate, forming a hood over the sorus, but shriveling with maturity, beneath sorus on midrib side, sporangia: brown to black, maturity: midsummer to early fall.
Habitat: on shaded rock and cliff faces but also occasionally on forest floors .
Distribution: central to northeastern North America, disjunct in Arizona, Utah.
Hardy to -30°C, USDA Zone 4.
Closest to C. protrusa and C. fragilis, differing from both in having pinnae commonly angled upward, some also curving towards the tip of the frond. Differs from C. protrusa in having the fronds clustered at the end of the rhizome, short-creeping rather than long-creeping. Differs from both (less so from C. protrusa) in having margins rounded rather than serrate.
Nephrodium tenue Michaux
Cystopteris fragilis (Linnaeus) Bernhardi var. mackayi G. Lawson