Bull. Calif. Acad. Sci. 1: 72. 1885.
Plants sparsely to densely pubescent proximally, sometimes 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.
Phenology: Flowering Feb–Jun.
Habitat: 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
Elevation: 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.