Thallus: verrucose-areolate, 2-10 cm in diam., 0.2-0.7(-2) mm thick areoles: rounded to sometimes angular or irregular, ±convex, (0.2-)0.4-1.2(-2) mm in diam., contiguous, separated by ±large cracks, or ±dispersed, especially at the edge prothallus: fimbriate, often present at the thallus edge but also in more central parts of the thallus, then visible between dispersed areoles (sometimes also with areoles growing directly on it), blue-black to brown-black or black at the margin, pale in central parts, (0.3-)1-2(-4) mm wide, sometimes even wider surface: brown, sometimes olive-brown, gray-brown or ochre, rarely olive, often with white to gray spots, lines or rims; ±shiny upper cortex: 20-40(-50) µm thick, uppermost part ±brown, 5-15(-20) µm thick, with cells 5-7(-10) µm in diam.; cortex covered with an epinecral layer 2-12(-22) µm thick photobiont: chlorococcoid, cells ±round, 5-16(-19) µm in diam. Apothecia: aspicilioid, usually rather common, (0.1-)0.3-0.8(-1.3) mm in diam., 1(-4) per areole, round, rarely angular; fertile areoles often somewhat elevated and almost entirely occupied by the apothecia, making them appear lecanorine disc: black, concave to plane, rarely convex, without pruina or with a ±white pruina thalline margin: flat to ±elevated, sometimes prominent, concolorous with thallus or with a white to gray rim exciple: (20-)30-70(-100) µm wide, sometimes clearly visible as a dark line surrounding the disc, I- or sometimes partly I+ blue medially; uppermost cells brown, ±globose, (4-)5-6(-9) µm in diam. epihymenium: green to olive or olive-brown, without or with a few crystals, rarely with abundant crystals, N+ green to blue-green, K+ brown hymenium: hyaline, I+ persistently blue, (90-)110-150(-170) µm tall paraphyses: moniliform, with (2-)3-6(-8) upper cells ±globose, (3-)4-5(-6) µm wide, in lower part 1.5-2 µm wide, slightly branched and anastomosing subhymenium and hypothecium: pale, I+ persistently blue or rarely partly turning yellow-green, together (15-)30-60(-70) µm thick asci: clavate, (60-)70-110 x (18-)20-33(-40) µm, (6-)8-spored ascospores: hyaline, simple, ellipsoid, (16-)18-26(-27) x (9-)10-16(-18) µm Pycnidia: usually rather common, 1-2(-10) per areole, immersed, mostly with a white to gray rim, rarely aggregated, (110-)130-200(-240) µm in diam., with a black, punctiform to sometimes elongated ostiole, 50-100(-120) µm in diam. conidia: filiform, straight or slightly curved, (10-)14-20(-25) x (0.6-)0.8-1(-) µm Spot tests: cortex and medulla I-, K-, P-, C- Secondary metabolites: none detected by TLC. Substrate and ecology: on ±exposed, siliceous or volcanic rock; montane World and Sonoran distribution: California (south and southeastern part, in El Dorado, Inyo, Los Angeles and Mono Counties) and Baja California, at 1000-3410 m. Notes: Aspicilia nashii is a distinct species, characterized by a verrucose-areolate, brown thallus, an often rather prominent, fimbriate prothallus, distinct apothecia often appearing lecanorine, moniliform paraphyses, rather large ascospores, rather long conidia, and by the absence of secondary substances. Aspicilia americana, with a more eastern distribution, differs from A. nashii by its mottled pattern of gray and brown on the thallus, its contiguous, flat areoles, and by its lack of the fimbriate prothallus seen in A. nashii. Aspicilia desertorum might superficially also resemble A. nashii, but lacks prothallus, has 2-4(-6) spores per asci, shorter conidia, and usually contains aspicilin.