Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Cal nearly regular, 5-nerved, the lobes equal or the upper the largest; cor bilabiate, the upper lip entire or 2-lobed, galeate-hooded, the lower spreading, constricted at the base of the enlarged, cleft or emarginate central lobe, the lateral lobes broad and low, seemingly borne on the cor-throat; stamens 4, ascending under the upper lip, the lower pair the longer; pollen-sacs conspicuously hairy, ± divergent or divaricate; nutlets trigonous, truncate above; herbs with toothed or pinnatifid, mostly cordate lvs and white to red or purple fls in verticils of 6–12 subtended by scarcely reduced lvs. 40, temp. Eurasia and n. Afr.