Thallus: placodioid, rosulate, rimose-areolate, mostly forming discrete rosettes or low mounds, up to 2 cm in diam., sometimes confluent; mostly 1-2 mm thick; prothallus: absent areoles: somewhat convex, usually contiguous with each other and with marginal lobes, but sometimes discrete lobes: flat to somewhat convex or undulating, 1.5-2(-3) mm wide, 2-3(-5) mm long, 0.3-0.5 mm thick, appressed, often becoming loosely adnate and free from the substrate over one third of their length, contiguous to slightly overlapping; tips: thinner (c. 0.2 mm), flat to undulating or downward curving, often slightly arched up off substrate; edges: rounded, entire to sinuous or deeply crenate-incised, thin, +downward curving upper surface: usually pale to moderate yellowish green or greenish yellow, the edges often dark olive-brown to olive-black, dull to slightly shiny, epruinose, usually strongly but finely rimose centrally, without thallospores, esorediate upper cortex: with dead algal cells, c. 50 µm thick, inspersed with yellowish granules (soluble in K), with elongated, variously oriented hyphae 3-5 µm in diam. and with 1-2 µm wide lumina; epinecral layer c. 10 µm thick medulla: solid, subcartilaginous, moderately loose, with randomly oriented hyphae 3-5 µm in diam.; algal layer: somewhat interrupted by a few +indistinct conical hyphal bundles, 50-75 µm thick lower surface: greenish black, smooth, even, continuous lower cortex: with thin dark green walls and hyphae 5-7 µm thick, overall c. 30 µm thick at near lobe tips, in upper part short, branched and embedded in a hyaline matrix, in lower part extending as loose rhizoidal hyphae up to 50 µm long Apothecia: scattered to crowded, borne laminally to submarginally on inner parts of lobes, broadly adnate to sessile, slightly constricted at base when mature, 0.5-1.5(-2) mm in diam. disc: mostly light orang-ish yellow to light to moderate yellow-brown (when young and especially next to margin often light grayish olive to medium or dark olive brown), plane to soon at least moderately convex, smooth, slightly to strongly shiny, epruinose margin: concolorous with thallus, entire to sinuous or coarsely crenate, 0.1-0.2 mm wide, soon even with disc, finally crowded back, cracked, with an indistinct, olivaceous parathecial ring amphithecium: present, in structure similar to the thallus parathecium: hyaline, with agglutinated, elongate, thick-walled, +parallel hyphae 2-3 µm in diam. and with 1 µm wide lumina epihymenium: densely inspersed with fine yellow-brown granules (mostly soluble in K), 10-15 or more µm thick hymenium: hyaline, 65-75 µm tall; paraphyses: strongly coherent; tips: enlarged, up to 3-5 µm wide; subhymenium: inspersed with oil droplets; hypothecium: hyaline, up to 200-300 µm thick centrally; with +randomly oriented hyphae asci: clavate, 8-spored ascospores: hyaline, simple, (8-)9-11(-12) x (4-)5-6(-7) µm, ellipsoid to broadly ellipsoid or ovoid-ellipsoid Pycnidia: immersed; ostioles: pale; conidiophores: type III of Vobis (1980) conidia: +curved, 20-30 µm long Spot tests: thallus and apothecia K-, C-, KC-, P- (yellow below); olivaceous or blackened parts N+ weakly reddish; cortex KC+ yellow, P-; medulla K-, C-, KC-, P+ yellow Secondary metabolites: cortex with usnic acid (major); medulla with psoromic acid chemosyndrome, and the murolic acid fatty acid complex (with neodihydromurolic acid major). Substrate and ecology: on hard siliceous rocks (rhyolite, solidified volcanic ash), in full sun or shaded, Sonoran Desert, thorn forest, desert grassland, or pine-oak woodland, at 580-2000 m World distribution: southern to northwestern Mexico Sonoran distribution: Sonora. Notes: Lecanora sonorae appears to be rather constant in its morphology and chemistry, and once recognized, it cannot easily be confused with other species. Lecanora sonorae is distinguished from the species for which it is most likely to be mistaken, L. muralis, by the former's broad and +convex lobes, rimulose surface toward the thallus center, mostly pale yellow to orange discs, and P+ yellow reaction of the medulla.