More study is needed. In recent decades, the concept of fragilis has become limited to relatively northern (circumboreal) tetraploids derived from crossing of C. reevesiana Lellinger and an unknown parent, presumed extinct; see also tenuis. A coll. from TRIM (KY) has been annotated "probably fragilis x reevesiana" by Haufler, and fragilis was mapped in Ky. by Haufler et al. in FNA 2. There are also colls. from MORG (MO) and LAUR (EKY) that may be this species, and there are records from the mountains of Va. and N.C. (W). C. fragilis differs from protrusa and tenuis as follows (W): its leaf blades are relatively elongated (l/w ca. 3-4 versus 2-2.5), with pinnae usually perpendicular (versus ascending); its pinnules have serrulate margins (versus crenulate), and the basal ones are strictly sessile with a rounded to truncate bases (versus often short-stalked, with rounded to cuneate base); its indusia are lanceolate, up to 1 mm long (versus ovate to round, ca. 0.5 mm).