Thallus: foliose, ± loosely appressed, up to 5 cm in diam. lobes: flat, usually distinctly concave towards the lobe tips, rarely convex throughout, 0.7-1 mm wide upper surface: gray, greenish gray, almost white or stramineous; pseudocyphellae: sparse, laminal and marginal, usually restricted to the peripheral parts of the lobes, sometimes reticulately confluent; pruina: patchy, rarely punctiform, often glistening; soralia, isidia and polysidiangia lacking medulla: white throughout lower surface: black in center, paler towards lobe tips; rhizines: ± dense, blackish, furcately divided Apothecia: cocoes-type, often present, laminal, 0.5-1.5 mm wide; disc: black, not pruinose internal stipe: distinct, upper part rose-red or orange-red (rarely white: P. petricola var. convexula - not yet known from the Sonoran region), lower part white ascospores: one-septate, brown, 14-19 x 5-7.5 µm Pycnidia: immersed conidia: bacilliform, 3-4 x 1 µm Spot tests: upper cortex K-, C-, KC-, P-; medulla upper and lower part K-, C-, KC-, P-; internal stipe upper part K+ blackish purple or purple, C-, KC-, P-; lower part K-, C-, KC-, P- Secondary metabolites: upper cortex with lichexanthone; medulla with few terpenes in low concentration. Substrate and ecology: on bark, wood and acidic rock from sea level to montane areas World distribution: pantropical with scattered collections from the subtropics Sonoran distribution: southern Arizona, Baja California, Baja California Sur, Sonora and Sinaloa, particularly common at low elevations in thorn forests. Notes: Populations with a stramineous and more closely appressed thallus with convex lobes have been described as Physcia devertens Nyl., Énumér. Lich. Annobon: 3 (1896) = Pyxine devertens (Nyl.) Vain., Add. Lichenograph. Antill. illustr.: 70 (1915), but examination of the holotype (H-Nyl. 31792) showed it to be identical, including the chemistry (no norstictic acid could be demonstrated by HPTLC, see however Swinscow and Krog 1975: 61) with typical P. petricola except for the differences mentioned above.