Thallus: crustose, well developed, continuous or cracked-areolate, rarely dispersed, ecorticate; prothallus: prominent, white or bluish gray surface: lemon-yellow to yellowish white or rarely deep yellow, esorediate Apothecia: immersed initially, eventually becoming almost sessile, 0.5-1.5(-2.5) mm in diam. disc: black, flat to somewhat convex or even hemispherical, epruinose margin: black, persistent but soon becoming excluded, usually white pruinose, but typically obscured by the expanding disc, without a parathecial ring amphithecium: absent parathecium: externally black but internally hyaline except dark blue-green at outer edge (N+ red) epihymenium: yellow-green to dark blue-green or greenish black, N+ red hymenium: hyaline, 55-75 µm tall, paraphyses: 1.5-2.5 µm wide, simple or sparsely branched and anastomosed; apices: brown, swollen, capitate, 3.5-5(-6) µm wide; subhymeium: hyaline; hypothecium: hyaline, 80-200 µm thick asci: clavate, 8-spored ascospores: hyaline, simple, narrowly to broadly ellipsoid, (6.5-)9-14 x (4-)4.5-6(-7.5) µm Pycnidia: frequent conidia: vermiform to arc-like, 11-25 x 0.5-1 µm Spot tests: cortex K+ yellow, C-, KC+ yellow, P-; medulla P- or pale yellow, K- or K+ pale yellow, C-, KC-, P- Secondary metabolites: usnic acid and atranorin, both variable in concentration. Substrate and ecology: on hard, calcareous or base-rich rocks (sometimes weakly calciferous siliceous rocks), in exposed alpine or arctic sites World distribution: Holarctic-alpine of Europe, North America and Australia Sonoran distribution: Arizona. Notes: Lecanora marginata differs from L. oreinoides (Körb.) Zopf in having larger apothecia and a yellowish thallus containing usnic acid. In contrast, L. oreinodes differs in having a much more distinctly areolated thallus with aspicilioid sunken apothecia. Lecanora marginata differs from other lecideoid species in occurring on calcareous or base-rich rocks, having +pure black discs, and containing atranorin and usnic acids only. For example, Lecanora subimmersa (Fée) Vain. occurs on acidic substrates, has brown discs, and lacks usnic acid.