Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Cal short, with 8–12 setaceous teeth; cor 4-lobed, ± irregular, the marginal ones distinctly so; involucral bracts about equaling the head; receptacle densely hairy, without bracts; involucel 2-toothed at the top, 4-ribbed; herbs with fls in dense hemispheric heads on long peduncles. 60, Europe, w. Asia, n. Afr.