Thallus: placodioid, forming +flattened rosettes 0.5-1(1.5) mm across, 1.5 mm thick centrally, tightly adnate throughout, areolate, rosulate; prothallus: absent areoles: contiguous, 0.2-0.5 mm across lobes: contiguous but discrete, +distinctly elongated, up to 1 mm long, 0.2-0.5(-1) mm wide, crenate to incised, +convex; edges not differentiated; tips dropping off abruptly to the substrate upper surface: pale greenish yellow, appearing yellowish white (to blue or gray tinged when parasitized), densely pruinose throughout, less so near lobe tips, which become rimose, esorediate upper cortex: +with dead algal cells, 100 µm thick, the upper 65 µm inspersed with grayish granules (insoluble in K) in addition to yellow-brown granules (insoluble in K) medulla: filled with gray granules (pol+, partly soluble in K) throughout; algal layer: +continuous, 65-75 µm thick lower surface: white near lobe tips, otherwise not visible lower cortex: absent Apothecia: common in thallus center, 0.5-1 mm in diam., sessile, soon constricted at base disc: yellowish to orangish brown, dull, uniform in color, +plane, epruinose margin: concolorous with thallus, 0.1-0.2(-0.3) mm wide, raised then level, soon strongly crenate towards the inside, persistent, with a +gray-green parathecial ring amphithecium: present, with an interrupted algal layer marginally and extending below the hypothecium, with loosely arranged medulla composed of branched and anastomosing hyphae c. 2.5 µm wide and with elongated lumina 0.5-1 µm wide (boundaries of both very distinct), and with fine hyaline granules (insoluble in K) in places, corticate; cortex: composed of radiating hyphae with roundish cells 4-5 µm in diam. and inspersed with yellow-brown granules (partly soluble in K) along the walls, and with grayish granules (pol+, insoluble in K), c. 60 µm thick parathecium: +hyaline, with strongly conglutinate, periclinal hyphae c. 3 µm wide and with narrow (1-1.5 µm), elongated lumina epihymenium: inspersed with yellow-brown granules (partly soluble in K), with c. 7.5 µm superficial uninspersed layer, c. 20 µm thick hymenium: hyaline, c. 65 µm tall; paraphyses: strongly conglutinate, indistinct even in K; tips (in K) 3-4 µm, hyaline; subhymenium: indistinct; hypothecium: with randomly oriented to vertically oriented hyphae and fine, hyaline granules insoluble in K asci: clavate, 40 x 15 µm, 8-spored ascospores: hyaline, simple, ellipsoid to broadly ellipsoid, (5-)7.5-10 x 4-6 µm Pycnidia: immersed, the ostioles flush with the thallus, concolorous with the apothecial discs; conidiophores: type III of Vobis (1980), branched once or twice near apex, the basal cells 10-15 x 3-5 µm conidia: filiform, 10-17 µm long Spot tests: thallus K-, C-, KC, P- Secondary metabolites: cortex with usnic acid; medulla with roccellic acid. Substrate and ecology: on gentle to vertical N-faces towards tops of large sandstone outcrops, in grassland, 1975 m; associated lichens include Lecanora valesiaca (var. valesiaca and var. sibirica) and Acarospora heufleriana, which are both very abundant in the area World and Sonoran distribution: Arizona, Coconino Co., known only from the the region around Meteor Crater. Notes: Lecanora arenisaxicola belongs to section Pruiniferae s. lato of the subg. Placodium and differs from L. valesiaca, with which it grows side by side at the type locality, by the following features: thallus thickerareoles distinctly convex; lobes +convex, with level edges and tips sharply set off from the substrate (dropping off abruptly); upper surface becoming rimose and more weakly pruinose towards the lobe tips; apothecia rather sparse, soon constricted sessile, with thicker margins soon strongly crenate towards the inside; cortex of thallus and apothecia thicker, +with dead algal cells. It differs from L. kofae and L. neodegelii in having shorter and narrower lobes, thicker cortex and algal layer, and from L. phaedrophthalma in having smaller apothecia with plane, uniformly colored discs, and it differs from all of these species in having a densely pruinose upper surface with grayish granules (insoluble in K) within the cortex. It comes closest to the central Asian species Lecanora usbekica Poelt, but differs especially in that the thallus center is verrucose-areolate (rather than rimose-areolate), the apothecial margins are soon strongly crenate towards the inside, and the cortex of the thallus and apothecia is thicker.