Thallus: crustose, areolate, thin; prothallus: vestigial, black areoles: scattered to contiguous, plane to +convex, 0.5-1 mm wide, with entire to crenate-incised edges upper surface: pale yellowish green, grayish greenish yellow, or moderately yellow, the edges usually blackening (but without thallospores, at least in material from the Sonoran region), dull to glossy, sometimes pruinose towards inside, esorediate upper cortex: with dead algal cells, 5075 µm thick, inspersed with yellowish granules (soluble in K); hyphae: thick-walled, 5-8 µm diam., with 1-2 µm wide lumina; cells: often in chains, isodiametric and rounded ones on black edges of areoles, olivaceous to dark green; epinecral layer: 10-40 µm thick medulla: dense to rather loose, 200-400 µm thick, with granules (partly insoluble in K), with 3-7 µm wide hyphae; algal layer: continuous to interrupted, 50-100 µm thick lower surface: pale to blackened lower cortex: absent Apothecia: common, adnate to sessile and +constricted, 0.5-1.5 mm in diam. disc: dark grayish brown, moderate brown, or pale yellowish brown to deep yellow, epruinose or more often thinly and finely pruinose margin: paler than thallus, partly blackened towards the outside (with thallospores?), scarcely raised, entire to flexuous or coarsely crenate, somewhat flattened on top in vertical section, 0.1-0.2 mm wide, slightly pruinose, without a parathecial ring amphithecium: present, with a well developed, +continuous algal layer (50 µm thick) marginally and extending under the hypothecium, with a medulla having whitish granules (fine ones are insoluble in K and N), and with reddish granules (K+ yellow, dissolving in K), corticate; cortex: similar to that of thallus but often bluish black and non-inspersed in outer part, 40-50 µm thick parathecium: hyaline, with thick-walled, strongly gelatinized, conglutinated hyphae 3-5 µm wide and with 3-10 x 0.5-1 µm lumina epihymenium: reddish brown to yellowish brown, 10-15 µm thick, inspersed with +fine granules (soluble in K), covered by 5-10 µm thick irregular superficial layer of hyaline granules (partly insoluble in K; insoluble in N) hymenium: hyaline, 40-60 µm tall; paraphyses: strongly gelatinized; tips: somewhat enlarged, 24 µm in diam., yellowish brown to olivaceous above (olive in K; N-); subhymenium: grayish brown, 75-100 µm thick; hypothecium: hyaline, with +randomly oriented hyphae asci: clavate, 8-spored ascospores: hyaline, simple, ellipsoid to ovoid, 8-12 x 4-5(-6) µm Pycnidia: sometimes common, with black and slightly raised ostioles; conidiophores: type III of Vobis (1980) conidia: filiform, 20-25 µm long Spot tests: thallus and apothecia K-, C-, KC-, P-; cortex KC+ yellow, blackened parts K+ green, N+ red-violet; medulla KC- Secondary metabolites: cortex with usnic acid, and sometimes placodiolic or isousnic acids or both; medulla with traces of fatty acids, and sporadically occurring phenolic substances (including norstictic and stictic acids, probably contaminants from other lichens). Substrate and ecology: on acidic rock (usually granite or volcanic rocks), often on steep and somewhat shaded surfaces, possibly parasitic on other lichens when young, mostly montane, in open pine forests, big cone Douglas fir and oak forest, or pinyon-juniper woodlands World distribution: western North America Sonoran distribution: southern California, 1375-2500 m. Notes: The disc pruina on Lecanora semitensis is often very inconspicuous or occasionally absent, and some specimens show resemblances to poorly developed forms of species that have epruinose discs and are usually distinctly radiating. For example, specimens containing placodiolic acid might be confused with poorly developed forms of L. phaedrophthalma, which differs in having mostly convex discs with apothecial margins thin, pale and becoming excluded; most specimens identified in herbaria as "L. muralis v. semitensis" or (Californian material only) "L. muralis v. diffracta" (or equivalents) are actually L. mellea, which almost always forms coherent, distinctly lobed thalli. Lecanora semitensis as delimited here is rather variable, and material from our region differs from the more common morphotypes in often having pale-edged areoles and apothecial margins, sometimes epruinose with rather pale yellow discs, and sometimes more narrowly ellipsoid (4-5 µm wide) spores. Material of L. semitensis from the Sonoran region tends to have densely pruinose apothecial margins and often lacks the typical blackening of the areole edges.