Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Cal 2-lipped, strongly 13-nerved, angular, sparsely villous, the upper lip flattened, oblong, with 3 conspicuous nerves prolonged into very short, broadly triangular lobes, the lower lip about as long, cleft nearly to the base into narrow, subulate- tipped lobes; cor-tube up curved, the upper lip concave, 2-lobed, the lower lip with rounded lateral lobes and a longer, oblong or obovate median lobe; stamens 4, ascending under the upper cor-lip and about equaling it; nutlets obovoid, smooth. 3, Eurasia.