Thallus: placodioid, orbicular, up to 8 cm across, to 0.25-0.5 mm thick, usually tightly adnate throughout, rimose to areolate or verrucose-areolate, at the margin radiate-plicate areoles: plane to +convex, contiguous, discrete to confluent, 0.5-1 mm across, often +angular in outline lobes: contiguous, confluent, tightly adnate or occasionally +imbricate, 3-5 mm long, 0.5-1.5 mm wide, c. 0.25 mm thick, divided and transversely broken by partial cracks, +plane, towards the tips rounded, broadened, and crenate upper surface: usually dark gray (greenish- to olivaceous- or olive-gray, or grayish brown) to black, sometimes paler (yellowish gray to cream-colored or gray-white) at the periphery, unchanged wet, smooth and epruinose, or sometimes +spottily pruinose, often irregularly maculate in places; extreme lobe tips often olive-black upper cortex: phenocortical, with +abundant dead algal cells, but also with +anticlinal hyphae with +round cells, c. 30 µm thick; upper 10 µm brown, with abundant fine dark brown granules and with hyaline, coarsely granular and plate-like crystals (all granules and crystals insoluble in K, appearing diffuse gray); epinecral layer: absent medulla: dense, without hyphal bundles, with coarse and fine hyaline granules and plate-like crystals (all insoluble in K-); algal layer: up to 150 µm thick, algae grouped, below upper cortex lower surface: not visible lower cortex: absent Apothecia: usually numerous, mostly densely crowded centrally, long remaining immersed, finally adnate to broadly sessile, 0.5-1.5 mm in diam., often compound or confluent and irregular in outline disc: red-brown to gray-brown, brownish black or almost black, at first concave to almost urceolate, then plane, usually epruinose margin: 0.1 mm wide, entire to very irregular, scarcely raised; parathecial crown: conspicuous or not; cortex: absent or up to 20 µm wide, phenocortical (but appearing paraplectenchymatous); with dark fine granules (insoluble in K); algae: scattered in margin and below hypothecium; medulla: with hyaline, coarsely granular and plate-like crystals (both insoluble in K) epihymenium: 10-15 µm thick, without fine dark granules soluble in K, sometimes (in pruinose discs) with coarse hyaline granules (insoluble in K), without plate-like crystals; covered by hyaline gelatinous layer hymenium: hyaline, 70-80 µm tall; paraphyses: non-monilliform (subapical cells long, straight), tightly compacted, with globose apical cells 2-3 µm wide; hypothecium: c. 30 µm thick asci: oblong-elipsoid, c. 45 x 15 µm ascospores: hyaline, simple, short- to long-ellipsoid, 11-15 x 6-8.5 µm Spot tests: thallus K- or indistinctly brown, C-, KC-, P- Secondary metabolites: none detected but norstictic acid reported by Hermann et al. (1973) from Europe and Israel; early reports of psoromic and salazinic acids based on var. subcircinata (Eigler and Poelt 1965). Substrate and ecology: on hard siliceous rocks (basalt and other volcanic rocks, granite, rhyolite) or silicate-carbonate rocks (silica content almost equal to that of calcium or magnesium) in upper Sonoran Desert to pinyon-juniper-Ponderosa pine transition areas World distribution: Europe, Asia, and North America Sonoran distribution: central to eastern Arizona at 670-2120 Notes: Lobothallia radiosa differs from other members of the genus by its apothecia being +persistently immersed, its frequently concave and often shiny discs, its simpler internal structure, and smaller asci. According to Eigler (1969), the taxa described (from Europe) as Lecanora. subcircinata Nyl., and L. velebetica (Zahlbr.) Kusan, now usually treated as a synonyms of Lobothallia radiosa) differ from typical L. radiosa in several ways, including at least the medulla being K+ red. These and other segregate specific or infraspecific taxa with K+ red thalli are apparently lacking in the Sonoran region, but can be confusable with L. praeradiosa; see under that species for general morphological comparisons. Within the K- material of L. radiosa s. lato there are a number of variants, some of which have been given published names that have been ignored by more recent authors. In the Sonoran region, there are several main variations distinguishable by combinations of the color patterns, surface texture and pruinosity of the thallus and apothecia, the conspicuousness of the parathecial crown, and the size, discreteness, form and adnation of the areoles, lobes, and apothecia.