Thallus: crustose to almost squamulose, verrucose-areolate or dispersed-areolate, verrucose or verruculose; prothallus: not visible areoles: thin or thick, opaque, ecorticate surface: yellowish white to yellowish gray or whitish gray to gray, smooth, epruinose, with an indistinct margin or effigurate, esorediate Apothecia: subimmersed when young, sessile when mature or sessile, 0.4-1.5 mm in diam., lecanorine disc: red-brown, plane, epruinose margins: concolorous with thallus, thin or thick, persistent, even, entire or flexuose, smooth, entire or verrucose or verruculose, without a parathecial ring amphithecium: present, with numerous algal cells, with large crystals insoluble in K, corticate; cortex: hyaline, indistinct or distinct, basally not thickened, gelatinous or interspersed, 1525 µm thick parathecium: hyaline, containing crystals soluble in K epihymenium: without crystals, red-brown to orange-brown, with pigment not dissolving in K hymenium: hyaline, clear; paraphyses: red-brown to orange-brown, slightly thickened or capitate (up to 4.5 µm wide) apically; subhymenium: hyaline, 15-20 µm thick; hypothecium: hyaline, without oil droplets asci: clavate, 8-spored ascospores: hyaline, simple, ellipsoid, (8-)8.5-13.5(-14) x (5-)6.2-7.5(-8) µm; wall: less than 1 µm thick Pycnidia: immersed, cerebriform; conidiophores: of type II sensu Vobis conidia: filiform, 15-18 µm long Spot tests: thallus and apothecial margin K+ yellow, C-, KC-, P- or P+ pale orange Secondary metabolites: containing atranorin (major), chloroatranorin (minor), 2'-O-methylhyperlatolic acid (minor or absent), 2'-O-methylisohyperlatolic acid (minor), 2'-O-methylperlatolic acid (major) and 2'-Omethylsuperlatolic acid (minor). Substrate and ecology: very common on exposed siliceous rocks, rarely on soil World distribution: subcosmopolitan, known from all continents except Antarctica Sonoran distribution: Arizona, southern California, Baja California, Baja California Sur, Chihuahua, Sinaloa, and Sonora. Notes: Lecanora pseudistera is the most common saxicolous species of the L. subfusca-group in the Sonoran Desert region. It is characterized by red-brown apothecial discs, an egranulose epihymenium, large amphithecial crystals, and the presence of the 2'-O-methylperlatolic acid chemosyndrome. It is the only species in the L. subfusca-group where well developed thalli may become squamulose. Several similar species are discussed by Brodo (1986) and Lumbsch (1998). This cosmopolitan taxon has mostly been confused with L. campestris, probably because that is the most common taxon of this group in Europe. This species, however, is easily distinguished in having small amphithecial crystals and lacking depsides. The differences to the similar L. galactiniza are discussed under that species.