Thallus: crustose, dispersed-verrucose to verruculose; prothallus: not visible areoles: flat to verrucose, thin or thick, opaque, ecorticate surface: yellowish white to pale yellow or whitish gray, smooth, epruinose, with a distinct margin, esorediate Apothecia: sessile, 0.3-0.8(-1) mm in diam., lecanorine disc: orange-brown to wax-colored, epruinose margin: concolorous with the thallus, thick, persistent, smooth, entire or verruculose, without a parathecial ring amphithecium: present, with numerous algal cells, with large crystals insoluble in K, corticate; cortex: hyaline, more or less gelatinous, basally slightly thickened, interspersed with small crystals, 20-35 µm thick laterally and 30-45 µm thick basally parathecium: hyaline, containing crystals soluble in K epihymenium: yellowish brown, with numerous coarse crystals, crystals and pigmentation soluble in K hymenium: hyaline, clear; paraphyses: not thickened (up to 1.5 µm wide) apically; subhymenium: hyaline, 15-20 µm thick; hypothecium: hyaline, without oil droplets asci: clavate, 8-spored ascospores: hyaline, simple, ellipsoid, 11-15.5 x 6.5-9 µm; wall: less than 1 µm thick Pycnidia: not seen Spot tests: thallus and apothecial margin K+ yellow, C+ orange-yellow, P+ yellowish orange Secondary metabolites: atranorin (major), arthothelin (major), thiophaninic aid (major), chloroatranorin (minor), 2,4-dichloronorlichexanthone (minor), 4,5dichloronorlichexanthone (minor), isoarthothelin (minor) and thiophaninic acid (minor or absent). Substrate and ecology: on bark World distribution: in tropical and subtropical regions of Australasia and North America Sonoran distribution: Chihuahua, Sinaloa and Sonora. Notes: Lecanora arthothelinella can be distinguished by the orange-brown apothecial discs, the relatively large ascospores, and the presence of chlorinated xanthones. Lecanora helva also has pale apothecial discs, but that species has smaller apothecia, broader ascospores and a different thallus chemistry. L. guderleyii is also similar, but differs in having a leprose thallus and apothecia with thin margins. These are the first records for the species from North America.