Thallus: areolate, sometimes rimose or verrucose, up to 15-20 cm in diam., (0.2-)0.4-0.6(-1.4) mm thick areoles: angular to irregular, flat to convex or uneven, (0.2-)0.4-1(-1.5) mm in diam., contiguous, separated by distinct cracks prothallus: absent surface: blue-gray, rarely gray to white-gray or brown-gray, dull upper cortex: (15-)25-30(-35) µm thick, uppermost part brown to yellow-brown, in K turning ±rose-brown, 5-10(-12) µm thick, with cells 4-7 µm in diam.; cortex sometimes covered by an epinecral layer, 3-6(-10) µm thick photobiont: chlorococcoid, with ±round cells 7-16(-21) µm in diam. Apothecia: aspicilioid, usually numerous, (0.1-)0.4-1(-1.3) mm in diam., 1(-5) per areole, round to angular disc: black, concave to sometimes finally plane, without pruina thalline margin: flat to ±elevated, sometimes prominent, concolorous with thallus exciple: (25-)40-50(-60) µm wide, usually I+ blue but sometimes I-; uppermost cells olive to olive-brown, ±globose, 5-6 µm in diam. epihymenium: olive to olive-brown, without crystals, N+ green to blue-green, K+ brown hymenium: hyaline, I+ wine-red to yellow-green, (85-)120-170(-200) µm tall paraphyses: sub-moniliform, with 1-2 upper cells ±globose to subglobose, 4-5 µm wide, lower cells broadly ellipsoid to oblong, in lower part 1.5-2 µm wide, slightly branched and anastomosing subhymenium and hypothecium: pale, I+ persistently blue or partly turning yellow-green, together 30-50 µm thick asci: clavate, 100-110 x 20-25 µm, (4-)6(-8)-spored ascospores: ellipsoid, sometimes globose when young, often poorly developed, (17-)20-25(-30) x 13-14(-16) µm Pycnidia: rare to rather common, 1(-3) per areole, 100-200 µm in diam., with a black, punctiform ostiole, 50-100 µm in diam conidia: filiform, straight, 6-10(-12) x (0.8-)1 µm Spot tests: cortex and medulla I-, K-, P-, C- Secondary metabolites: aspicilin. Substrate and ecology: on siliceous rock, usually in open situation World distribution: temperate to boreal to arctic regions of Europe and North America Sonoran distribution: not found in the Sonoran area, but a bit north of it, in Tulare County, California, at 1365 m. Notes: Aspicilia caesiocinerea is characterized by a blue-gray thallus, a tall hymenium, submoniliform paraphyses, large spores, short conidia and the presence of aspicilin. The name A. caesiocinerea has been widely but incorrectly used for various specimens from the Sonoran area, for example specimens belonging to Aspicilia americana, A. cinerea, A. confusa and A. cyanescens. They all differ from A. caesiocinerea by having moniliform paraphyses, longer conidia and by lacking aspicilin. Aspicilia caesiocinerea has not been found in the Sonoran area, but farther north in California (Sequoia National Park, Tulare County). The specimen at this locality (Wetmore 50159; MIN) lacks mature spores, but shows the other typical features of A. caesiocinerea, for example the short conidia and the presence of aspicilin. Another specimen from the same locality (Wetmore 50179; MIN) also contains aspicilin, but the paraphyses are moniliform and the conidia longer (up to 27 µm), and therefore is rather similar to A. cyanescens, which however lacks aspicilin. This specimen needs further study.