Thallus: areolate, rarely rimose, (1-)2-6 cm in diam., (0.1-)0.2-0.5(-0.8) mm thick areoles: angular, sometimes irregular, flat to slightly convex, (0.2-)0.4-1.0(-2.0) mm in diam., contiguous, at the thallus edge sometimes sub-radiate with extended, slightly raised areoles prothallus: often present but narrow, at the thallus edge, rarely fimbriate, brown-black to blue-black or black, 0.2-0.5(-1.0) mm wide surface: gray to green-gray, rarely olive-gray or brown-gray, sometimes in a mosaic pattern on the areoles, dull, rarely slightly shiny upper cortex: (15-)20-40(-45) µm thick, with ±crystals, uppermost part ±brown, 5-15 µm thick, with cells 5-7(-8) µm in diam.; cortex covered with an epinecral layer 2-12 µm thick photobiont: chlorococcoid, cells ±round, 5-18 µm in diam. Apothecia: aspicilioid, usually rather numerous, (0.1-)0.2-0.8(-1.5) mm in diam., 1-2(-7) per areole, usually round, sometimes angular disc: black, usually concave, sometimes plane, epruinose or rarely with a thin white pruina thalline margin: usually flat to slightly elevated, rarely prominent, concolorous or darker than thallus, rarely with a thin white to gray rim exciple: (20-)30-60 µm wide, I- or partly I+ blue medially, rarely entirely I+ blue; uppermost cells brown to olive-brown, ±globose, (4-)5-6 µm in diam. epihymenium: green to olive or olive-brown, rarely brown, without or with a few crystals, N+ green to blue-green, K+ brown hymenium: hyaline, I+ persistently blue, (130-)150-200 µm tall paraphyses: moniliform, rarely submoniliform, with (1-)3-7(-9) upper cells ±globose, (3-)3.5-5(-6) µm wide, in lower part 1.5-2(-3) µm wide, slightly branched and anastomosing subhymenium and hypothecium: pale, I+ persistently blue, together 40-70(-100) µm thick asci: clavate, 60-100(-110) x 20-30 µm, 8-spored ascospores: hyaline, simple, ellipsoid, (15-)19-26(-29) x (9-)11-16(-21) µm Pycnidia: rare to rather common, 1(-3) per areole, immersed, rarely elongated, (80-)120-150 µm in diam., with a black, punctiform ostiole, (50-)60-90(-110) µm in diam. conidia: filiform, (8-)10-16(-18) x (0.6-)0.8-1µm Spot tests: cortex and medulla I-, K+ red, P+ orange C- Secondary metabolites: norstictic acid, often also with trace of connorstictic acid. Substrate and ecology: on siliceous or volcanic rock World and Sonoran distribution: Arizona, at 1070-2900 m. Notes: Aspicilia arizonica is characterized by a gray to green-gray thallus, a tall hymenium, moniliform paraphyses, large spores (usually over 20 µm), rather short to medium long conidia and the presence of norstictic acid. Aspicilia arizonica might resemble A. cinerea, which also contains norstictic acid, but this species has a short hymenium and smaller spores. The type specimen and another specimen from the same locality (Owe-Larsson 8763 & 8762) have been analyzed by DNA (ITS). These studies show that A. arizonica is related to, but clearly separated from, Aspicilia knudsenii. The latter species has a brown thallus color, and usually contains stictic acid as a major substance and norstictic acid as a minor one. Two specimens from shaded habitat in Calf Pen Canyon, Gila County (Schultz 16137; Zschau 48) have a thin, partly rimose thallus but in other respects agree well with A. arizonica. Some specimens with a yellow-gray to ochre thallus color resemble A. arizonica microscopically but it is at present uncertain if they belong to this species. These specimens have been found in Apache Co. (Nash 7830 & 27023), Gila Co. (Nash 25763) and Santa Cruz Co. (Nash 18553).