Herbs, annual or biennial, prostrate. Leaves opposite, pinnate. Flowers axillary, solitary. Sepals 5. Petals 5, yellow, imbricate. Disk ring-shaped. Stamens 10, with glands, outer 5 longer and opposite petals, inner 5 shorter and alternate with petals. Fruit a schizocarp of 4 or 5 indehiscent prickly carpels. Seeds obliquely pendulous; endosperm thinly membranous.

About 15 species: mainly in the temperate zone; two species in China.