Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Sep and lateral pet much alike, spreading or reflexed; lip declined or projecting horizontally, in our spp. much longer than the lateral pet, 2-lobed or 2-cleft; column erect, shorter than the pet, the anther borne on its back near the tip; pollinia 2; stem bearing a single pair of broad, opposite, sessile lvs near its middle and a terminal raceme of small, green to purple or red fls; our spp. glabrous below the lvs, ± pubescent above them. 25, N. Hemis.