Thallus: orbicular to irregular, up to 2 cm diam. or bigger when confluent with other thalli, ± loosely adnate lobes: radiating, flat to slightly convex, narrow, up to 1 mm but rarely exceeding 0.5 mm, sinuose and truncate, eciliate upper surface: whitish gray to cream-colored, mostly without pruina, sorediate soredia: in prominent, marginal soralia, situated at some distance from the lobe tips, delimited, lip-shaped to semicapitate, sometimes abundant upper cortex: paraplectenchymatous medulla: white lower cortex: proso- to indistinctly paraplectenchymatous with thick walled cells, but thinner thalli are frequently with paraplectenchymatous lower cortex lower surface: white to grayish, rarely brownish, with few pale rhizines Apothecia: ± common, up to 1 mm diam.; margins: usually dissolved into soredia; disc: sometimes pruinose ascospores: brown, 1-septate, Pachysporaria-type, 15-19 x 6.5-7.5 µm Pycnidia: not common, immersed conidia: subcylindrical, 4-6 x 1 µm Spot tests: cortex and medulla K+ yellow, C-, KC-, P+ yellow Secondary metabolites: upper cortex with atranorin; medulla with atranorin, zeorin and an unknown triterpene. Substrate and ecology: growing on rocks in ± open situations World distribution: occurring in a few localities in Brazil and Venezuela Sonoran distribution: only known from widely scattered localities at relatively low elevations from Arizona, Baja California Sur, Sonora and Sinaloa. Notes: The species is easily recognized by the sinuose and truncate lobes and the well delimited soralia situated in the lobe angles. Superficially it resembles P. stellaris, but differs by the presence of soralia and the K+ yellow medulla. Physcia rolfii has a similar type of soralia but the lobes are bigger and convex and the the upper surface is maculate. Physcia sinuosa is also similar to P. nashii but the latter is smaller and lacks zeorin in the medulla.