Thallus: verruculate, overall up to c. 2 cm wide verrucae: convex, 0.3-1 mm wide, 0.3-0.5 mm thick, dispersed or in small contiguous groups; rim: narrow, down-turned upper surface: brown, dull, convex, smooth, epruinose upper cortex: paraplectenchymatous, 10-15(-40) µm thick; eucortex: upper half pigmented dark brown and c. 5 µm thick, lower layer hyaline, often indistinct algal layer: filling most of the verrucae, interwoven with hyphae medulla: prosoplectenchymatous, white, varying in thickness and usually reduced, continuous with rhizohyphae lower surface: narrow, subcorticate, pale green from algal layer or pale brown attachment: broad, with rhizohyphae, sometimes thickening, but without a stipe Apothecia: one per areole, often lecanorine, 0.3-0.7 µm wide disc: brownish red, turning red when wet parathecium: indistinct epihymenium: reddish brown, 8-30 µm thick hymenium: hyaline, 110-120 µm tall; paraphyses: 1-1.7µm wide at base, spreading in K, apices not expanded subhymenium: colorless and indistinct, 15-20 µm thick; hypothecium: indistinct asci: clavate, c. 83 x c. 17 µm, (16-)24- or more spored ascospores: hyaline, simple, ellipsoid, 10-12 x 5-7 µm Pycnidia: not seen Spot tests: UV-, all spot tests negative Secondary metabolites: none detected. Substrate and ecology: on granite or sandstone, usually on pebbles or stones, occasionally on boulders, often solitary, lower elevations up to 2130 m World distribution: Northern Europe and North America Sonoran distribution: Arizona (north rim of the Grand Canyon) and southern California. Notes: Acarospora oligospora is probably under-collected. Some verrucae of A. veronensis and A. badiofusca in southern Californian mountains resemble A. oligospora, but those specimens have smaller spores and a hundred or more spores per ascus. Acarospora oligospora is similar to A. thelococcoides and A. macrospora in having large spores and a polyspore count per ascus usually below 64 spores. It differs from A. thelococcoides in occurring on rocks. It differs from A. macrospora in its preference for acidic rocks and in usually having larger spores. Large-spored specimens should be checked with KC. Acarospora oligyrophorica Aptroot, described from Brazil, is a member of oligospora-macrospora group and contains gyrophoric acid (Aptroot 2002).