Thallus: areolate, ±orbicular, subradiate, several specimens often growing adjacent to each other, up to 3(-6) cm in diam., 0.5-1(-1.5) mm thick areoles: angular to irregular or sometimes round, flat or sometimes convex, in central part of thallus usually fertile, (0.2-)0.5-1.3(-2) mm in diam., contiguous; at the thallus edge extended, 1-2(-3) x 0.5-1.2(-1.5) mm, usually sterile prothallus: thin, indistinct at the thallus edge, gray-black to brown-black or with a bluish tinge, up to 0.1(-0.2) mm wide surface: ochre to yellow-brown or white-gray to white, central parts often darker with a blue-gray tinge due to a parasymbiont, extended areoles at the thallus edge usually distinctly white; dull and sometimes ±farinose due to crystals upper cortex: 20-50 µm thick, sometimes indistinctly delimited from the crystal layer above, uppermost part brown, 10-15(-20) µm thick, with cells 5-7 µm in diam.; cortex covered with a Ca-oxalate crystal layer, (10-)20-50(-70) µm thick photobiont: chlorococcoid, cells ±round, 5-18(-22) µm in diam. Apothecia: aspicilioid, numerous to scattered in the middle of the thallus, often with a white or gray rim, (0.1-)0.3-0.8(-1.2) mm in diam., 1-2(-4) per areole, round to angular (or sometimes irregular or elongated), fertile areoles often somewhat elevated disc: black, concave or sometimes flat, with ±white pruina thalline margin: thin, ±elevated or flat, concolorous with the thallus or sometimes darker exciple: I-, rarely partly I+ blue, (20-)40-60(-110) µm; uppermost cells brown, ±globose, (4-)5-7(-9) µm in diam. epihymenium: green to olive or olive-brown, with crystals, N+ blue-green to green, K+ brown hymenium: hyaline, I+ persistently blue, (90-)100-130(-160) µm tall paraphyses: moniliform, with (2-)3-5(-6) upper cells ±globose, 4-5(-6) µm wide, in lower part c. 2 µm wide, not or slightly branched and anastomosing subhymenium and hypothecium: pale, usually I+ persistently blue, together (20-)30-50(-65) µm thick asci: clavate, 55-100 x (17-)19-28(-30) µm, 8-spored ascospores: hyaline, simple, ellipsoid to globose, (9-)11-15(-16) x 7-11(-12) µm Pycnidia: rare to rather common, 1-2(-10) per areole, immersed, sometimes aggregated in branched formations, up to 180-300 µm in diam., with a black, punctiform to elongated ostiole 70-150(-200) µm in diam. conidia: filiform, straight or sometimes curved, (12-)13-20(-30) x (0.8-)1(-1.5) µm Spot tests: cortex and medulla I-, K-, P-, C- (chemotype 1); K+ red, P+ orange, C- (chemotype 2); K+ yellow or red, P+ orange, C- (chemotype 3) Secondary metabolites: 3 chemotypes: 1. no substance detected by TLC (most of the specimens, including the type); 2. norstictic acid; 3. stictic, hyposalazinic, and norstictic (trace) acids. Substrate and ecology: on limestone or ±calciferous sandstone World and Sonoran distribution: northern Arizona, at 1890-2705 m, where most of the specimens has been collected in Grand Canyon National Park and extending to the SW corner of Colorado. Notes: Aspicilia boykinii is characterized by a ±orbicular, subradiate, ochre to yellow-brown or white-gray to white areolate thallus (which often has a blue-gray tinge in the middle due to a parasymbiont), green to olive (or olive-brown) epihymenium, rather short hymenium, small, ellipsoid to globose spores and filiform, rather long conidia. The species has three chemotypes. Aspicilia boykinii seems to be rather common in the Grand Canyon National Park, Arizona and adjacent areas. It shows some resemblance to Aspicilia determinata, but the latter species has a brown epihymenium, a continuous algal layer below the hypothecium and bacilliform conidia. Aspicilia cheresina (Müll. Arg.) Hue, an orbicular, calciferous species occurring in the Mediterranean region in Europe and North Africa, has the same chemical variation and small spores as A. boykinii, but differs by having bacilliform conidia, and is often found parasitizing Aspicilia calcarea (L.) Mudd. Aspicilia candida (Anzi) Hue, which according to Thomson (1997) is found in North America south to Colorado, is another light colored, calciferous species, but it differs from A. boykinii by having larger spores and submoniliform paraphyses.