Thallus: to 3 cm across, distinctly umbilicate, usually monophyllous but sometimes strongly lobed lobes: to 1.5-3 mm long, 2-3 mm wide, plane to slightly concave, entire to coarsely and shallowly crenate, ultimate segments: 0.5-1 mm wide, edges: not thickened, often rolled downward upper surface: ± pale greenish yellow to yellow, sometimes grayish tinged, turning brownish or orange in herbarium, continuous to strongly rimose, epruinose to partly pruinose, matt or slightly nitid, edges: concolorous or blackened, sometimes with thallospores lower surface: ± yellowish brown, to bluish black near edges, smooth to uneven, ± strongly cracked towards center, with medulla showing through Apothecia: usually common, to 2-3 mm diam., long remaining immersed to broadly adnate (crater-like with broadly sloping rims), or at least finally sessile; disc: concave then plane or sometimes convex, epruinose, orangish yellow to yellowish or reddish brown; thalline margin: c. 0.1-0.4 mm wide, slightly to moderately raised or later level, usually persistent, entire to flexuous or coarsely crenate towards outside, ± concolorous with thallus; hymenium: hyaline, deeply inspersed with brownish granules; paraphyses tips hyaline, clavate, ascospores: ± ellipsoid to subglobose, c. 9-12 x 5-8 µm Pycnidia: sometimes present conidia: 20-40 µm long Spot tests: upper cortex K-, C-, KC+ yellow, P-, UV-; medulla (in Sonoran region material) K-, C-, KC-, P+ orange, UV- Secondary metabolites: upper cortex: with usnic acid only (the report by Leuckert et al., 1976, of rare specimens with placodiolic acid, is dubious); medulla: with zeorin, and (in most sites in North America) pannarin, ± rangiformic acid or various unknowns; in other areas, specimens containing psoromic, lecanoric, norstictic, or hypoprotocetraric acid. Substrate and ecology: on rock (often weakly to somewhat strongly calciferous, or at least exposed to calcareous dust), often on vertical or steep surfaces, often N-facing, or slightly protected, moderately nitrophilous, in pinyon-juniper and oak-pine woodlands, on basalt, schist, sandstone, and limestone World distribution: southern Europe; SW to central Asia, northern Africa; temperate to boreal western North America Sonoran distribution: Arizona, 1100-2400 m; also present in California (at least the northern parts). Notes: Although frequently confused with R. chrysoleuca and R. melanopthalma, in the past, R. peltata is easily distinguished by the presence of terpenoids (and, in North America, almost always pannarin), and the discs are always epruinose and distinctly brownish, with the upper part of the hymenium densely inspersed. The cracked lower surface of thallus and the often crater-like apothecia are also helpful identifying characteristics. The illustration of R. peltata in Swinscow and Krog (1988, p. 291) is of a Lecanora species.