Mapping is provisional. Several records of this northeastern species are uncertain, or may be hybrids; there has also been confusion with tennesseensis. C. tenuis is a fertile tetraploid (2n = 168) derived from crossing of protrusa and another parent, unknown and presumed extinct (Paler & Barrington 1995). It is similar to protrusa but differs (W) in its shorter rhizomes, extending no morwe than 0.5 cm beyond petioles (versus 1-6 cm), which lack hairs (versus covered in hairs); petioles dark brown (versus darkened only at base); blades lanceolate to elliptic with ascending and often incurving pinnae (versus lanceolate to deltoid with almost perpendicular pinnae); distal pinnae ovate to narrowly elliptic (versus deltate to ovate); basal pinnules just slightly stalked and more or less cuneate (versus clearly stalked and more truncate) and just slightly lobed (versus deeply); pinnules generally thicker (versus membranaceous), with veins to teeth and notches (versus mostly teeth).