A genus of 3-4 species, annual herbs, of se. North America and Central America. The recognition of Hemianthus as separate from Micranthemum is uncertain and needs additional study. Lewis (2019) states "morphological data (corolla and stamen structures) can be interpreted as providing support for recognizing the genus Hemianthus…, as was done by J.K. Small (1933) and F.W. Pennell (1935)".