Plants sparsely to densely pubescent proximally, some-times glabrous distally. Stems (0.2-)0.5-3(-5) dm. Basal leaves: petiole 0.1-0.5 cm; blade oblanceolate, obovate, or oblong, 0.3-1.5(-2) cm × (1-)2-6(-8) mm, margins entire or dentate, apex acute to obtuse, surfaces uniformly pubescent, trichomes cruciform and 3-rayed, adaxially sometimes mixed with coarse, simple ones. Cauline leaves 1-6; blade similar and equaling or smaller than basal. Fruiting pedicels slender, (1.5-)2-4(-6) mm. Flowers: sepals 0.5-1 × 0.3-0.7 mm; petals oblong, 1.5-3 × 0.5-1 mm; filaments 0.4-0.6 mm; anthers (0.05-)0.1-0.2(-0.25) mm. Fruits usually orbicular to obovate, rarely broadly oblong, 1-locular, not twisted; valves thin, veins sometimes prominent, usually pubescent, rarely glabrous, trichomes simple, to 0.5 mm, hooked (sometimes restricted to margins, or mixed with smaller, sessile, cruciform or 3-rayed ones); septum absent; ovules 2(-4) per ovary; style 0.05-0.2 mm. Seeds 1, (0.8-)0.9-1.1(-1.2) × (0.5-)0.7-0.9(-1) mm. 2n = 26. Flowering Feb-Jun. Open grassy slopes, grassy glades in woodlands, chaparral, sandy and gravelly flats, flood plains, rock outcrops, cliffs and ledges, on limestone, serpentine, sandstone, granitic and basaltic substrates; 0-1800 m; B.C.; Ariz., Calif., Idaho, Mont., Nev., Oreg., Wash.; Mexico (Baja California). Records of Athysanus pusillus for Montana appear to be solely from Ravalli County, and for Nevada from Storey and Washoe counties.

Plant: Annual, small, leafy toward base; stem 5-35 cm, often prostrate; branches generally from near base, slender; hairs simple or 2-5-branched Leaves: basal 6-30 mm, 2-10 mm wide, entire or obscurely dentate, short-petioled, hairs coarse; cauline like basal, sessile, bases not lobed INFLORESCENCE: raceme, terminal, slender, 1-sided, open; bracts 0 Flowers: inconspicuous, bisexual; sepals 4, free, ± 1 mm, early deciduous; petals 1-2 mm, spoon-shaped, white, sometimes 0; stamens generally (2,4)6, generally 4 long, 2 short; ovary 1, superior, chambers generally 2, septum membranous, connecting 2 parietal placentas, style 1, stigma simple or 2-lobed Fruit: capsule, with 2 deciduous valves, sometimes breaking transversely or indehiscent, 2-2.5 mm wide, ± circular, flat parallel to septum, indehiscent; larger hairs flat, hooked, smaller hairs branched or 0; pedicel recurved, 1-4 mm; style < 0.3 mm, chamber 1; Seed 1, flat; embryonic root at edges of both cotyledons Misc: Grassy, open slopes, rocky outcrops; generally < 1525 m.