Annual (rarely perennial), decumbent, glabrous herbs. Leaves alternate; stipules small, scarious; bracts small, scarious. Flowers in compound dichasia, 5-merous, bisexual; sepals free; petals entire, equalling or longer than sepals; stamens 5; ovary 1-celled with 1 basal ovule, style 1, trifid above. Fruit an indehiscent, more or less trigonous achene enclosed by the persistent calyx; seed globose-ovoid, 3-ribbed, finely striate.
About 10 species, cosmopolitan; 1 naturalised in Australia.