Thallus: orbicular to irregular, up to 1 cm diam., often confluent with other thalli forming a larger aggregate lobes: up to 1 mm wide, convex to flat, usually broadest at the downwardly bent lobe tips, eciliate upper surface: white to grayish, often with a darker zone near lobe tips, weakly maculate, dull to shiny, sometimes with a white pruina, sorediate soredia: in marginal soralia on lobe tips, sometimes big and reflexed or semicapitate; soredia ±granular upper cortex: paraplectenchymatous medulla: white lower cortex: pale, prosoplectenchymatous often labyrinthiform in the lower parts lower surface: white with a rose tinge; rhizines: concolorous or darker than the lower surface, few and scattered Apothecia: rare, up to 2 mm diam.; margins: ± sorediate ascospores: brown, 1-septate, Pachysporaria-type (14-) 14.5-18.5 (-20.5) x 7.5-9 (-9.5) µm Pycnidia: ± common, immersed, appearing as black warts conidia: subcylindrical, 4-6 x 1 µm Spot tests: upper cortex K+ yellow, C-, KC-, P+ yellow; medulla K-, C-, KC-, P- Secondary metabolites: upper cortex with atranorin (sometimes extending into algal layer but not below); medulla with some traces of triterpenes but not zeorin. Substrate and ecology: growing on ± exposed, acid rocks at altitudes from c. 50 m to 1200 m World and Sonoran distribution: a species of low to intermediate elevations from Arizona, Baja California, Baja California Sur, Chihuahua, Sinaloa and Sonora. Notes: Physcia nashii is recognized by its small size, the convex lobes, the presence of terminal soralia, and the absence of zeorin. In most species of Physcia the absence of zeorin in the medulla medulla is connected with a K medullary reaction, but in the case of P. nashii and P neglecta a K+ reaction is detected in the medulla even though no zeorin is present. P. nashii differ from P. neglecta by the type of the soralia and by the narrower lobes. It is also similar to the nonsorediate P. convexella, and P. nashii could be regarded as the secondary species of P. convexella. To some extent P. nashii is similar to P. poncinsii, but the latter is usually larger, has wider lobes and a different chemistry as it contain zeorin in the medulla.