Description from Flora of China
Herbs annual or perennial, stoloniferous. Stems erect, nodes hairy, base sometimes woody. Leaves cordate-ovate to lanceolate, papery, margin dentate. Verticillasters few flowered, lax, in terminal or sometimes axillary racemes, sometimes 2-flowered; floral leaves leaflike, lanceolate, gradually reduced upward; bracteoles 2, small, subulate or nearly bristly. Calyx campanulate or tubular-campanulate, hairy, glabrous inside, 15-veined, dilated in fruit, 2-lipped; teeth ovate-triangular to lanceolate, 3 in upper lip, 2 in abaxial. Corolla purplish to purple, tubular, base narrower, gradually dilated to throat, not hairy annulate inside, 2-lipped; upper lip shorter, straight, apex emarginate or 2-lobed; lower lip long, 3-lobed, with middle lobe larger. Stamens 4, didynamous, included or posterior 2 slightly exserted, sometimes slightly complanate; anther cells 2, puberulent. Style slender, exserted, apex equally 2-cleft. Nutlets oblong to oblong-ovoid, glabrous.
About seven species: six in temperate to subtropical Asia, one in North America; five species in China.