Cystopteris fragilis var. mackayi G.Lawson
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Variety Description - Lvs less cleft, the pinnules more remote, the lowermost pinnules only slightly lobed and mostly cuneate at the base; indusium ca 0.5 mm, or nearly obsolete, usually entire; a little more southern, from N.S. and N.B. and s. Que. to Minn. and Nebr., s. to Va., s. Ill., and Mo., and in the mts. to n. Ga. (C. tenuis)