Thallus: crustose to placodioid, areolate-squamulose, indistinctly radiating, closely appressed, not easily removed intact; prothallus: absent areoles: scattered to contiguous or weakly imbricated, flat to slightly convex or undulating, with +downward-turning margins, irregular in outline, 1(-2) mm wide, sometimes rimosely divided into secondary areoles, 0.2-0.5(-1.5) mm thick, thinner toward edge, becoming crenate, narrowly to broadly attached centrally or more towards one side; edges: often becoming free from substrate lobes: isodiametric to moderately elongated, 1-2 mm long, 0.5-1.5 mm wide, simple or slightly branched, often strongly crenate, partly overlapping each other, flat to concave or undulate, 0.2-0.3 mm thick, appressed, or slightly ascending on side toward thallus margin upper surface: usually with distinctly grayish green to light olive-gray, paler and more gray towards thallus margin (in older herbarium specimens partly with pale yellow to pale orange tinges), often somewhat roughened, +dull, with patches of white pruina but mostly epruinose, esorediate upper cortex: with few to many dead algal cells, even, without cone-like hyphal bundles; inspersed with yellowish granules (soluble in K), 5-10(-15) µm thick medulla: solid, moderately loose, partly filled with clumps of gray granules, with hyphae 3-5 µm in diam.; algal layer: continuous to interrupted, 30-50(-75) µm thick lower surface: pale, +moderately yellowish brown, darker brown near attachment areas lower cortex: absent or poorly developed, with hyphae c. 5 µm in diam. and lumina 2 µm wide Apothecia: usually numerous and becoming crowded, borne submarginally to laminally, up to 3 per areole, adnate to broadly sessile, 1-1.3 mm in diam. disc: light to deep orange to moderate yellowish brown often darker and more olivaceous when young; often slightly concave, then plane to undulate, +densely white to pale yellowi pruinose (pruina appearing gray-yellow) margin: often somewhat paler than thallus, 0.1- 0.2 mm thick, soon irregularly foveolate, entire or somewhat crenate, becoming flexuous or distorted, sometimes distinct and +raised from the start, persistent, weakly pruinose, without a parathecial ring amphithecium: present, with an algal layer occupying central part of margin and forming a continuous, +uneven layer (40-75 µm thick) below the hypothecium, medulla at least basally with grayish granules (insoluble in K and N), corticate; cortex: yellowish to reddish brown, obscured by material insoluble in K, strongly gelatinous, 20-30(-40) µm thick above, 40-50 µm thick near base parathecium: hyaline, with parallel, +gelatinized, randomly oriented hyphae 3-5(-7) µm in diam. and with +short lumina 1 µm wide epihymenium: pale brown, weakly inspersed with fine granules (partly soluble in K), covered by 15-20 µm thick layer of coarse granules (soluble in K) hymenium: hyaline, 50-65 µm tall; paraphyses: hyaline, distinct and +loose; tips scarcely thickened, 2-2.5 µm; subhymenium: pale gray, distinct; hypothecium: hyaline to slightly yellow-brown, up to 300 µm thick in center asci: broadly clavate, 8-spored ascospores: hyaline, simple, ellipsoid to oblong-ellipsoid, 8-11(-13) x (3-)4-6(-7) µm Pycnidia: immersed; ostiole: pale; conidiophores: type III of Vobis (1980) conidia: curved, (15-)20-25 µm long Spot tests: thallus and apothecia K-, C-, KC-, P-; cortex KC+ pale yellow; medulla KC- Secondary metabolites: cortex with usnic acid (often only traces); medulla with fatty acids of the protolichesterinic group (allo-pertusaric and dihydropertusaric, constipatic, protoconstipatic and dehydroconstipatic acids), and sometimes traces of unknowns. Substrate and ecology: on volcanic rock near tree line with fir and bristlecone pine World distribution: North America Sonoran distribution: northern Arizona at 2680-3030 m. Notes: The two taxa in our area with which L. weberi is most likely to be confused are L. novomexicana s. lato and Rhizoplaca melanophthalma s. lato. All three taxa have pruinose apothecia and an upper cortex containing dead algal cells, and contain usnic acid and fatty acids of the pertusaric/constipatic group. Lecanora weberi differs from the other two species especially by its very thin cortex, more grayish, pale-edged, frequently imbricated squamule-like lobes, and poorly developed cortex on the lower side of the apothecial margin. Its marginal lobes are shorter, flatter and more loosely attached than those of L. novomexicana. Lecanora weberi differs from R. melanophthalma in lacking a distinct umbilicus and lower cortex and having apothecia that are adnate, with orangish to yellowish brown discs and scarcely prominent margins. The relationship between L. weberi and L. nigromarginata (found north of the Sonoran region) needs further clarification. Specimens with blackening (but smooth) areole edges, and ones containing psoromic acid, are tentatively included under L. weberi.