Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Cal turbinate, 5–10-nerved, the lobes prolonged into stiff slender spines, the 2 lower often slightly larger or deflexed; cor strongly bilabiate, the upper lip entire and galeately rounded, the lower spreading or deflexed, 3-lobed, with obcordate central lobe and oblong lateral lobes; stamens 4, ascending under the upper lip, the lower pair equaling or longer than the upper; pollen-sacs mostly parallel; nutlets triquetrous, truncate and hairy at the top; erect herbs with toothed or cleft, petiolate lvs and white to pink fls crowded in dense verticils subtended by bracteal lvs and by linear to subulate bracts; forming long, interrupted terminal spikes. 10, mostly temp. Eurasia.