in A. Gray et al., Syn. Fl. N. Amer. 1(2): 410. 1884.
Annuals, 20–180 cm (± scapiform); taprooted. Stems 1–3+, erect, branched distally, glabrous. Leaves mostly basal or subbasal (usually flat against soil), proximal cauline relatively few; sessile or petiolate (petioles winged); blades obovate, finely dentate (obtuse); distal cauline reduced to triangular scales. Heads in corymbiform arrays. Peduncles not inflated distally, sometimes bracteate. Calyculi of 4–6, triangular to lanceolate bractlets. Involucres cylindro-campanulate, 6–13+ mm diam. Phyllaries 10–15 in 1–2 series, lanceolate, ± equal, margins broadly hyaline, apices acuminate. Receptacles flat or slightly convex, smooth or minutely pitted, epaleate. Florets 30–60 (fragrant); corollas white, sometimes tipped with rose or purple (at least outer much longer than phyllaries). Cypselae whitish, subcylindric or weakly clavate, not beaked, bluntly 4–5-angled or -ribbed, ribs corky-thickened, faces glabrous or minutely puberulent; pappi 0. x = 9.
sw United States, nw Mexico.
Molecular phylogenetic studies by J. Lee et al. (2003) indicated that Malacothrix is polyphyletic and that Atrichoseris is basal to the subclade of Malacothrix that does not contain the type species of that genus. Atrichoseris differs in base chromosome number (x = 9) from the species of Malacothrix with which it is most closely grouped (x = 7). Atrichoseris differs from Malacothrix in having apically truncate cypselae that are wholly epappose.