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DescriptionThallus: crustose, continuous or rimose-areolate; prothallus: blackish to bluish brown; areoles: flat, thin, opaque, ecorticate; surface: yellowish white to yellowish gray or yellowish green or pale green to greenish white, smooth, epruinose, with an indistinct margin, esorediate; Apothecia: sessile, 0.3-0.9 mm in diam., lecanorine; disc: orange-brown or yellowish brown, plane or convex, epruinose or slightly whitish gray pruinose; margin: concolorous with thallus, thin or thick, persistent or becoming excluded, even, not flexuose, smooth, entire or verruculose, without a parathecial ring; amphithecium: present, with numerous algal cells, with large crystals insoluble in K, corticate; cortex: distinct, basally not thickened, interspersed with numerous small crystals, hyaline, (15-)25-35(-40) µm thick laterally, (15-)25-35(-40) µm thick basally; parathecium: hyaline, containing small crystals insoluble in K; epihymenium: yellowish brown to orange-brown, with pigment dissolving in K, with crystals dissolving in K; hymenium: hyaline, clear; paraphyses: sparingly branched and slightly thickened (up to 2.5 µm wide) apically, not pigmented; subhymenium: hyaline, 15-20 µm thick; hypothecium: hyaline, without oil droplets; asci: clavate, 8-spored; ascospores: hyaline, simple, ellipsoid or broadly ellipsoid, 10.5-16.5 x 6.5-8.5 µm; wall: less than 1 µm thick; Pycnidia: not observed; Spot tests: K+ yellow, C- or C+ orange, P+ pale yellow; Secondary metabolites: arthothelin (minor or absent), atranorin (submajor), chloroatranorin (minor), 2'-O-methylperlatolic acid (major), usnic acid (major) and traces of unidentified terpenes.; Substrate and ecology: on the bark of deciduous trees; World distribution: pantropical occurring in North, Central and South America, Indian Ocean islands, Australia, north island of New Zealand, Papua New Guinea and the South Pacific; Sonoran distribution: Baja California Sur and Sinaloa.; Notes: Lecanora achroa is characterized by the comparatively small apothecia with orange-brown disc, the small ascospores, and the presence of usnic acid. It is similar to L. helva and L. leprosa, but readily distinguished by the different chemistry.