Thallus: areolate to subsquamulose, contiguous in indeterminate groups to several cm wide areoles: 0.3-5 mm in diam., 0.5-1 mm thick, round to angular, sometimes becoming lobed and imbricate upper surface: shades of brown, reddish brown, or with a grayish hue cortex: paraplectenchymatous, 40-85 µm thick; syncortex: indistinct, up to 25 µm thick; eucortex: 40-60 µm thick, with thin dark brown pigmented upper layer, with cells mostly round 5-7 µm in diam algal layer: even medulla: subprosoplectenchymatous, continuous with attaching hyphae lower surface: white, narrow, corticate attachment: broad Apothecia: usually one, but sometimes two or more per areole, round to irregular and confluent disc: pale red when dry, but sometimes darker, (0.2-)0.5-1 mm in diam, epruinose, plane, often with reddish coloration around the disc on the thallus parathecium: c. 20 µm thick, not expanded around disc epihymenium: reddish brown, c. 15 µm thick hymenium: hyaline, 100-160 µm tall; paraphyses: 2-3 µm wide at base, apices expanded up to 6 µm wide subhymenium: hyaline, 40-50 µm thick; hypothecium: c. 15 µm thick asci: clavate, 60-90 x 18-24 µm, 40-80-spored ascospores: hyaline, simple, broadly ellipsoid, 8-12 x 4-5 µm, often globose in early development Pycnidia: with a dark ostiole, 160-240 x 100-150 µm (Magnusson 1929b), sometimes absent conidia: almost globose, c. 1 x 0.7-0.9 µm (Magnusson 1929b), not seen in North American specimens Spot tests: UV-, all spot tests negative Secondary metabolites: none detected. Substrate and ecology: on carbonate rock or on decaying granite rocks in washes World distribution: Europe, North America (Arkansas, California, Missouri) Sonoran distribution: expected to occur as it was recently collected just outside the Sonoran area in the Granite Mountains of the Mojave Desert in California. Notes: Its areoles, wider paraphyses, preference for carbonate substrates, and usually smaller spores easily differentiate Acarospora macrospora from A. oligospora. It is apparently infrequent in North America. The taxon was named many times in Europe, leading to a confusing number of synonyms.