PLANTS: Shrubs and subshrubs; hairs barbed, not stinging. LEAVES: sessile or petiolate; blades ovate, lanceolate, elliptic or linear; margins entire, crenate or dentate. INFLORESCENCE: racemes. FLOWERS: subtended by two small bracts and often nearly hidden by the third and outer bract; calyx persistent or irregularly deciduous, the lobes linear or lance-linear; petals distinct or the adaxial edges of the claws connate such that the stamens are held outside the tube at anthesis, deciduous, white to cream, rarely pale yellow; stamens 5, exceeding petals, all fertile (in ours), or 3 lacking anthers; filaments linear, distinct, free; pistil 3-carpellate; stigma minute. FRUIT: an achene. SEED: subapical; testa non-sculptured; endosperm lacking. X = 23. NOTES: 5 spp.; sw US; Baja C., s to c Mex. (Greek: petalon = petal + onyx = claw). REFERENCES: Christy, Charlotte M. 1998. Loasaceae. J. Ariz. - Nev. Acad. Sci. 30(2): 96.