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 similar; lip strongly saccate in the basal half, the terminal lobe broadly ovate, not hinged to the basal part, crested at its base with 2 evident swellings; column very short and broad, the large sessile anther hinged to its back below the middle and somewhat projecting; pollinia 4; rhizomatous or stoloniferous plants with numerous alternate lvs and a terminal raceme. 25, N. Hemis.