Life habit: lichenized Thallus: peltate-subumbilicate to ± areolate in appearance but all attached together; thalli c. 1-3 mm wide, 0.3-0.8 (-1.2) mm thick, usually monophyllous, attached with a broad, central stipe, appressed, rarely almost crustose, often with deep cracks between them; sometimes lobate lobes: broadly rounded, up to 5 mm, partly ascending, concave or slightly convex, the margins rolled under upper surface: white or bluish white when dry, reddish brown when moist, spottily to densely pruinose upper cortex: paraplectenchymatous, 40-50 µm thick, grayish, the outer 7-11 µm reddish or yellowish brown; cells indistinct, thick-walled, 2-3(-5) µm diam.; epinecral layer irregular, up to 50 µm thick medulla: chalky, densely filled with calcium oxalate crystals; hyphae: very loose, arachnoid, thick-walled, 3.5-4.5 µm wide, becoming thicker towards the lower side photobiont: primary one a Trebouxia, secondary photobiont absent; algal layer: 50-75 µm thick with ± continuous layer, algae 8-14 µm diam lower cortex: lacking lower surface: pale, sordid whitish to somewhat brownish, or light yellowish brown near the margin, matt, smooth to finely powdery-roughened, cracked here and there, without rhizines; umbilicus: very wide Ascomata: apothecial, common, compound, forming multiple structures, the individual apothecia punctiform to circular or elongate, each with a proper exciple; with a pruinose net between, 0.7-1 (-2.5) mm diam.; disc: epruinose, scabrid, red-brown to dark brown, becoming slightly higher than the thallus surface; cortex between discs: similar in structure to that of thallus; 'exciple': composed of parallel hyphae, I+ blue, 20-25 µm thick; epihymenium: yellow-brown; hymenium: 100 µm, hyaline, I+ blue; hypothecium: 40-50 µm thick, I+ blue; paraphyses: in water rather indistinct, with coherent tips, in HCl distinct, septate, 2-2.5 um wide, the tips clavate, 3.5-4 µm; algal layer below the apothecia strongly interrupted asci: clavate, Acarospora-type, with tholus I+ slightly blue, without axial structures, without distinct ocular chamber, 55-65 x 15-17µm, c. (50-) 100-spored ascospores: spherical, 3.5-4 µm diam.; wall: hyaline, thick Pycnidia: very rare, 150 µm deep, 100 µm wide, simple, flask-shaped; conidiophores 10 µm long conidia: c. 2 x 1 µm Spot tests: upper cortex and medulla K-, C+ red, KC+ red (at least upper medulla), P- Secondary metabolites: gyrophoric acid (an orcinol tridepside). Substrate and ecology: in arid to semi-arid regions, on calcareous slate and siliceous limestone, on steep surfaces, ± exposed, usually on the end surface of larger boulders World distribution: Arctic to cool temperate regions; Europe, northern Africa; southern Asia, North America Sonoran distribution: Arizona, on limestone, 1900-2230 m (several collections from the Grand Canyon; see Boykin and Nash  for specimen data). Notes: The compound apothecia and the subfoliose, umbilicate thallus distinguish this genus from Acarospora. In a sense the species is part of a transition group between the crustose growth form and foliose; the lack of a lower cortex and multiple attachment structures (e.g. rhizines) argues against calling it foliose.