Thallus: areolate to verrucose, (0.5-)1-6 cm in diam., (0.1-)0.2-0.7(-2) mm thick areoles: angular to rounded or irregular, flat to finally ±convex, (0.2-)0.4-1.5(-2.3) mm in diam., contiguous and separated by distinct cracks, or sometimes ±dispersed, especially at the thallus edge prothallus: rarely present, then very sparsely developed along the thallus edge, fimbriate or forming a narrow dark zone, black to brown-black or olive-black, 0.1-0.4(-0.7) mm wide surface: brown to olive-brown, sometimes partly olive, ochre, gray-brown, gray or gray-white (from pruina), dull to ±shiny upper cortex: (15-)20-45(-70) µm thick, uppermost part ±brown or rarely olive-brown to green, 5-15(-30) µm thick, with cells (4-)5-7(-9) µm in diam.; cortex covered with an epinecral layer or crystals, (2-)5-22(-40) µm thick photobiont: chlorococcoid, cells ±round, 5-20(-28) µm in diam. Apothecia: aspicilioid, usually rather common to common, (0.1-)0.2-0.7(-1.4) mm in diam., 1-2(-5) per areole, round to sometimes angular, elongated or irregular disc: black, concave, rarely flat, usually with a white pruina thalline margin: flat to usually ±elevated and prominent in older apothecia, usually with a white to gray rim, sometimes concolorous with thallus exciple: (15-)20-60(-100) µm wide, I+ fully blue or partly I+ blue medially, sometimes also I-; uppermost cells brown, ±globose, 5-7(-9) µm in diam. epihymenium: olive-brown to brown, sometimes olive, rarely green, with crystals, N+ green to blue-green, K+ brown to green-brown hymenium: hyaline, I+ persistent blue, (100-)130-190(-220) µm paraphyses: submoniliform to moniliform, with (1-)2-4(-6) upper cells ±globose to sometimes subglobose, rarely subcylindrical,, (3-)4-6(-7) µm wide, in lower part (1-)1.5-2(-3) µm wide, slightly branched and anastomosing subhymenium and hypothecium: pale, I+ persistently blue, together (20-)30-60(-80) µm thick asci: clavate, (60-)80-130(-165) x (17-)20-32(-40) µm, 2-4(-6)-spored ascospores: hyaline, simple, globose to subglobose, (14-)19-32(-36) x (13-)16-26(-28) µm Pycnidia: rare to rather common, 1-2(-5) per areole, immersed, (80-)120-180(-250) µm in diam., rarely aggregated, with a black, punctiform to rarely elongated ostiole, usually surrounded by a white rim, (40-)50-100(-120) µm in diam. conidia: filiform, straight or slightly curved,, (5-)6-11(-13) x 0.8-1(-1.5) µm Spot tests: cortex and medulla I-, K-, P-, C- Secondary metabolites: usually with aspicilin, rarely lacking secondary metabolites (14 % of the specimens from the Sonoran area). Substrate and ecology: on siliceous rocks, boulders, or small stones on the ground, sometimes on volcanic, calciferous or calcareous rock, in deserts, chaparral, scrubland or open areas with scattered trees World distribution: Eurasia; North America (U.S.A.: south part of the Great Plains and mid-west, west to California) Sonoran distribution: Arizona, southern California, Sonora and Baja California, at 240-2170 m. Notes: Aspicilia desertorum is characterized by a brown to olive-brown or gray-brown thallus, usually lacking a prothallus, with contiguous to dispersed, finally ±convex areoles. Furthermore, the disc of the apothecia usually has a white pruina and the thalline margin a white rim, the asci have 2-4(-6) globose to subglobose spores, the conidia are short, and aspicilin is usually present. Two other species with aspicilin and 2-6 globose to subglobose spores per ascus, A. contorta and A. praecrenata, occur in the Sonoran area. Aspicilia contorta differs from A. desertorum by its white to gray or green-gray, rather thin thallus with ±dispersed, flat areoles, A. praecrenata, by being terricolous and having apothecia with a ±crenulate thalline margin. Aspicilia calcarea (L.) Mudd. has not been found in the Sonoran area, although the name has frequently been used for North American material. That European species differs from A. desertorum by its white to gray, contiguous, rimose-areolate thallus, delimited by a prothallus, and the lack of aspicilin. European specimens of Aspicilia desertorum, including the type material, usually have a thick thallus with large apothecia, and the algal layer, especially in the thalline margin, is arranged in glomerules interrupted by hyphal tissue. Furthermore, they lack aspicilin. Some specimens from Idaho, Nevada and New Mexico in USA (including the type of Lecanora elmorei, see synonym above) agree with these. The Sonoran specimens of A. desertorum usually have thinner thallus and the algal layer is not arranged in glomerules (but intermediate specimens exist), and they usually contain aspicilin. Three representative specimens from the Sonoran area (Riverside County, CA: Knudsen 2046; Yavapai County, AZ: Owe-Larsson 8759 & 8770) have been analyzed for DNA (ITS, unpublished results). So far we have no sequences from European material. Such sequences are needed before the relationship between the Sonoran and European populations can be settled.