Acarospora fuscata (Schrad.) ArnoldThallus crustose, episubstratic, areolate, often forming extensive, up to 10 cm wide patches. Areoles 0.2-2(-3) mm wide, up to 1 mm thick, mostly contiguous, irregular in outline, usually flat and uneven, epruinose, pale to reddish brown, often with black margins and undersides. Epicortex thin (< 10 μm thick) or absent; cortex 25-35(-50) µm thick, yellow-brown in uppermost part, otherwise colourless, paraplectenchymatous, with perpendicularly arranged cells and crystals visible under polarized light; algal layer continuous, 50-100 μm thick; medulla white, 0.1-0.8 mm thick, of intricate, thin-walled hyphae. Apothecia 1-5(-10) per areole, 0.1-0.4(-1) mm across, at first punctiform, later slightly expanded, with a concave to flat, usually angular or irregular, immersed, smooth to rough, reddish brown to blackish disc, and an inapparent thalline margin. Proper exciple distinct, 10-20(-30) μm wide, not expanding around the disc; epithecium brownish, 10-15 μm high; hymenium colourless, (70-)100-120(-140) μm high, hemiamyloid, IKI+ red or blue turning red; paraphyses coherent, mostly simple, (1-)1.5-2 μm thick at mid-level, the apical cells barely expanded or 2-4 μm wide; subhymenium 30-60 μm high, IKI+ blue (euamyloid); hypothecium 10–20 μm high. Asci 100-200-spored, clavate, the apical dome K/I-, 60-90 x 15-18 μm. Ascospores 1-celled, hyaline, narrowly ellipsoid to subcylindrical, 4-5(-6) x (1-)1.5-2 μm. Pycnidia immersed, the conidiogenous cells ampulliform. Conidia 1-2 x 0.5-1 μm. Photobiont chlorococcoid. Spot tests: cortex K-, C+ red, KC+ red, P-, UV-; medulla C-, KC-. Chemistry: cortex with gyrophoric acid.
