Perennials; caudex well-developed, often branched, somewhat thickened. Basal leaves: blade margins entire or toothed, without distinct terminal lobes. Petals 4-6 mm. Fruits 1.2-1.8 mm wide; style 0-0.5 mm. 2n = 16, 32. Flowering Jun-Jul. Sand dunes, sand bars, tussocks, gravel, tundra, volcanic ash, stream terraces; 0 m; Yukon; Alaska; n Europe; n, e Asia. The occurrence of subsp. petraea in North America was pointed out by S. L. O´Kane and I. A. Al-Shehbaz (1997) and Al-Shehbaz and O´Kane (www.aspb.org/publications/arabidopsis, 2002); no specimens were cited. G. A. Mulligan (1996), who treated the taxon as Arabis media, did not cite specimens either. Representative Alaskan collections were examined, including Parker 7995 (MO) and Hardy 134 (CAS, MO).