Thallus: thinly areolate, indeterminate, overall up to 6 cm wide areoles: usually sharp-edged, angular, up to 0.5 mm in diam., 0.1-0.3 mm thick, imbricate; rim: plane, ±black, ±down-turned upper surface: dark brown, dull, smooth, fissured or not, epruinose upper cortex: paraplectenchymatous, (10-)15-30(-40) µm thick; cells: round to irregular, 3-5 µm wide; syncortex: very thin or absent; eucortex: upper layer brown and c. 10 µm thick, lower layer hyaline and 10-30 µm thick lateral cortex: thinner than upper cortex algal layer: ±100 µm, algal cells usually ±10 µm in diam., forming an uneven stratum, the algal stratum interrupted by hyphal bands medulla: white, prosoplectenchymatous lower cortex: lacking or narrow around attaching hyphae, up to ten cells thick beneath rim lower surface: white or pigmented brown or black attachment: broadly attached, never forming a stipe or areoles becoming elevated Apothecia: usually one per areole disc: usually dark brown when dry, 0.1-0.2(-0.5) mm in diam.; punctiform parathecium: c. 10 µm thick, sometimes visible at surface as thin margin around disc, almost non-existent in smallest apothecia epihymenium: pale yellow to dark brown, c. 10µm thick hymenium: hyaline, 50-100 µm tall; paraphyses: (1.3-)1.5-1.7(-2.1) µm wide at base, apices usually not expanded; coherent in gel dissolving in K subhymenium: hyaline, gray, or pale yellow, 15-50 µm thick; hypothecium: indistinct, c. 10 µm thick asci: clavate, c. 70 x 10-15(-25) µm, 100+-spored ascospores: hyaline, simple, ellipsoid, (3-)4-5 x 1.5-1.7(-2.2) µm Pycnidia: not seen Spot tests: UV-, all spot tests negative Secondary metabolites: none detected. Substrate and ecology: on granitic and volcanic rocks; lichenicolous, observed growing out of the thallus of Aspicilia World distribution: Europe, North America, and Mexico Sonoran distribution: Arizona and southern California. Notes: The circumscription of A. scotica here does not follow the circumscription of Clauzade et al. (1981). Acarospora scotica looks like a Staurothele and is often sterile. It occurs in small amounts in many lichen communities and can often be spotted in packets of other genera. But it is rarely collected separately, except when its thallus is well-formed and extensive. It becomes dispersed on rough surfaces, leading to specimens Magnusson recognized as other species (see the synonyms given above). Sometimes it can be confused with immature thalli of A. badiofusca or other species in a young thin spreading areolate stage. Acarospora complanata H. Magn. is not considered a synonym of A. scotica or as occurring in North America. This author considers A. complanata a distinct C+ red species occurring in Europe, based on study of type (UPS!). The species needs further investigation.