Thallus: placodioid, up to 2-5(-10) mm thick centrally, loosely adnate (sometimes removable intact), usually rosette-forming (often rather irregularly and indistinctly so), areolate, becoming verrucose to lumpy on nitrogen-rich rocks; prothallus: absent, or occasionally present, black areoles: up to c. 1 mm in diam., discrete, +round, plane or convex, becoming crenate-lobed, often mixed with +elongated lobes lobes: 3-5(-7) mm long, 0.5-1(-1.5) mm across, 0.5-0.7 mm or more thick, usually distinctly elongated, radiating and separate, sometimes ascending, often +imbricate, plane to more often strongly convex or almost cylindrical, +richly incised-lobed upper surface: white to cream-colored, ashy gray or grayish brown, sometimes tinted ochraceous or rosy, sometimes becoming black centrally but usually concolorous throughout, occasionally maculate, smooth to +wrinkled or knobby, usually epruinose except spottily near the lobe tips upper cortex: eucortical, paraplectenchymatous, diffuse brown, inspersed with abundant dark brown granules (soluble in K), with coarse hyaline granules (insoluble in K), without coarse plate-like crystals; hyphae: mostly unoriented; epinecral layer: 15 µm thick medulla: loose to almost hollow, hyphal bundles present; hyphae: vertically arranged, with fine and coarse hyaline granules (insoluble in K), without large plate-like crystals; algal layer: algae grouped, becoming distributed in areas of optimal exposure to light lower cortex: present, paraplectenchymatous lower surface: pale or sometimes darker or more brown than upper surface, or mottled Apothecia: lecanorine, usually very numerous and crowded and deformed in thallus center, 0.4-1.5(-2.5) mm in diam., soon appressed-sessile to distinctly sessile or somewhat stalked disc: dark reddish brown to dark grayish brown, dark brown or black, plane or convex, often blue-white pruinose especially towards center, darkening when wet margin: +entire, +strongly raised and inflexed when young, 0.2-0.3 mm wide, often rosy colored next to disc; parathecial ring: conspicuous or not; cortex: eucortical, paraplectenchymatous, 20-40 µm thick, continuous with that of thallus; with numerous fine dark brown granules (soluble in K) along the cell walls; epinecral layer: present; algae: in groups in margin, absent from below hypothecium; medulla paraplectenchymatous; hyphae: thick, moderately swollen; with hyaline, coarse granular crystals (insoluble in K), without plate-like crystals epihymenium: diffuse pale brown, N+ lightly greenish brown, the pigment +dissolving in K; not inspersed, but with superficial fine or coarse hyaline granules (insoluble in K); without hyaline covering hymenium: hyaline, 7080 µm tall; paraphyses: coherent, 2.5 µm wide, the upper 3-4 cells submonilliform, with swollen, short, subglobose cells, the apical cell short-clavate, 2-3 µm wide, and hyaline, or capitate and 4.5-6 um thick, with thin gelatinous pigment cap; hypothecium: 30-40 µm thick asci: oblongelipsoid, 60-75 x 20-25 µm ascospores: hyaline, simple, 11-16 x 5-8(-10) µm, short-ellipsoid Spot tests: thallus and apothecia C-, KC-; cortex K+ red, P+ orange, or K-, P-; medulla K+ red, P+ orange Secondary metabolites: norstictic, constictic (9'-O-methylsalazinic acid), or salazinic acids. Substrate and ecology: on wide variety of siliceous rocks (e.g., basalt, pumice, and other volcanic rocks; schist, gneiss, quartzite, granite, sandstone, shale, conglomerate), occasionally spreading onto mosses; most frequent on base-rich siliceous rocks, often on weakly inclined, sunny surfaces in a wide range of communities World distribution: Europe, central Asia, and North America Sonoran distribution: Arizona, and Baja California. Notes: Lobothallia alphoplaca has most often been confused with L. praeradiosa, for comparisons see under that species. Another similar species (not found in our area) is L. melanaspis, which can be relatively easily distinguished by its K- thallus with upper surface turning green when wet and occurring in periodically flooded habitats. Following other authors, Lecanora thamnoplaca is listed as a synonym of Lobothallia alphoplaca, but here with a question mark, because according to Eigler (1969), that taxon differs from L. alphoplaca in a number of ways, especially in that the thallus cortical zone is phenocortical (with numerous dead algal cells) rather than eucortical (without dead algal cells).