Description from Flora of China
Herbs, relatively small. Stems terete or rarely quadrangular (Mazus lanceifolius), erect or procumbent and rooting from lower nodes. Leaves in a rosette or opposite, often upper leaves alternate; petiole winged. Racemes secund; bracts small. Bracteoles present or absent. Flowers small. Calyx funnelform or campanulate, 5-lobed. Corolla 2-lipped, palate with 2 longitudinal plaits; lower lip 3-lobed; upper lip 2-lobed. Stamens 4, didynamous, inserted on corolla tube; anther locules divergent, apically connivent. Ovary hairy or glabrous. Style glabrous; stigma 2-lamellate. Capsule compressed, included in cupular persistent calyx, loculicidal. Seeds small, numerous.
About 35 species: China, India, Indonesia, Japan, Korea, Malaysia, Mongolia, Philippines, Russia, Vietnam; Australia, New Zealand; 25 species in China.