Description. Thallus of areoles 0.3–2.5 mm wide, 300 μm thick, with mycelial base up to 500 μm thick, usually dispersed. Upper surface brown to black, smooth, some creases, sometimes densely pruinose. Lower surface white or absent. Epicortex usually 10 μm thick, sometimes extending across apothecial disc. Cortex 30–40 μm thick, upper layer dark brown, lower layer hyaline, variable in widths, cells round to irregular 2–7 μm. Algal layer 100–125 μm thick, algal cells 10–12 μm, scattered cells below apothecia, not interrupted. Medulla ca. 100 μm thick, hyphae 2–4 μm wide, continuous with mycelial base. Apothecia 0.2–1.0 μm wide, usually 1 per areole but rarely up to 5, immersed, epruinose, brown, red when wetted, sometimes with elevated parathecial crown or reduced to verrucae with parathecial margin. Parathecium 30–50 μm wide, sometimes forming an elevated perithecial crown around apothecia or expanding up to 100 μm to form verrucae, hyphae 1–2 μm wide, merging with cortex. Hymenium mostly 100–130 (–150) μm, epihymenium 10 μm thick, paraphyses mostly 2 μm wide, often filled with oil drops, apices not expanded in gel caps with black pigment mark, hymenial gel IKI+ blue to red, or red, hemiamyloid. Asci 80–90 × 20–30 μm, ascospores 3–4.5 × 1.5–2 μm, mostly thin. Subhymenium 20–40 μm IKI+ blue, sometimes bleeding into hymenial gel in sections in thin sections. Hypothecium narrow, 10 μm. Pycnidia not observed.
Chemistry. Not producing secondary metabolites. All spot tests and UV-reaction negative.
Distribution and ecology. Widespread in North America on a variety of non-calcareous rocks at usually lower elevations below 2000 m. The species also grows on wood, the only Acarospora that frequently does this in North America.
Discussion. Differs from A. veronensis A. Massal. in having a wider parathecium (30–50 μm vs. indistinct to 15 μm) and a thicker cortex [30–40 μm vs. usually 15–20(–30 μm)]. It differs in size depending on the habitat and sometimes it does not produce elevated parathecial crowns.