Thallus: areolate or squamulose, often overall up to 5 cm wide or in meandering single-file lines squamules: irregular, lobed, up to 2.5 mm in diam., up to 1 mm thick, contiguous; rim: down-turned, concolorous with surface upper surface: yellow, dull, plane to convex, fissured, rugulose, epruinose lateral cortices: subparaplectenchymatous, 50-70 µm thick; syncortex: not evident; eucortex: upper half opaque yellow, lower half: hyaline algal layer: upper surface even, lower surface somewhat uneven, c. 100 µm thick, becoming scattered beneath apothecium medulla: white, prosoplectenchymatous, continuous with attaching hyphae lower surface: ecorticate, white darkened by soil or mineral particles attachment: forming a stipe eventually Apothecia: often numerous, up to 4 per squamule, but often merging disc: dark red-brown, irregular in shape, beginning often as slits, often looking pruinose and dull with a fine layer of dissected sterile ascal plectenchyma embedded in the surface parathecium: not distinct epihymenium: yellow and opaque, with rusty-red pigment concentrated around dark apices of paraphyses, 20-25 µm thick hymenium: hyaline, conglutinated, 100-120 µm tall; paraphyses 1.7-2.3 µm in diam. at base, apically with scattered dark caps, variously expanded up to 4 µm wide, with some upper branching subhymenium: hyaline, 40-50 µm thick, hypothecium: not distinct asci: clavate, 60-70 x 20 µm, often rare, 100+-spored ascospores: hyaline, simple, broadly ellipsoid, usually c. 4 x c. 2 µm, perispore evident Pycnidia: not observed Spot tests: UV+ orange, medulla KC+ red, C+ red Secondary metabolites: acaranoic acid (major), gyrophoric acid (major), rhizocarpic acid (major), lecanoric acid (minor) (HPLC, J.A. Elix, pers. comm.). Substrate and ecology: on decomposing granite in full sun World distribution: North America Sonoran distribution: central Arizona. Notes: The spot test reaction is best seen under a microscope. Acarospora tuckerae is probably widespread and is apparently common in Missouri, Oklahoma, and South Carolina. So far southeastern specimens seen have been rather poorly developed. An important feature is the occurrence of acaranoic acid, a fatty acid found in Pleopsidium. The description needs revision once more specimens are studied.