Thallus: squamulose, forming lines along cracks in boulders, often growing among or on other lichens squamules: irregular, sometimes lobulate, with thick or poorly developed stipes, 0.3-1.5 mm in diam., up to 0.7 mm thick, sometimes contiguous and imbricate in domed clumps ±2 cm across, ±1.5 cm thick; rim: down-turned, often black upper surface: various shades of brown, even in same population, glossy, smooth, sometimes with blackish tones, often with fissures, epruinose lateral cortices: paraplectenchymatous to prosoplectenchymatous, 40-80 µm thick; cells: ±distinct in water, 2-3 µm wide; syncortex: c. 10 µm thick; eucortex: upper layer dark brown and 5-10 µm thick, merging into lower hyaline layer algal layer: even, sometimes feathering into medulla, appearing thinner with age medulla: white to dirty brown, opaque, prosoplectenchymatous lower surface: black, sometimes carbonized attachment: broad with medullary hyphae sometimes forming thick interconnected stipes up to 1.3 mm tall, often rudimentary stipes visible in vertical cross-section Apothecia: 1 or more per squamule, round but usually becoming very irregular, up to c. 0.5 mm wide disc: usually black, very rough, even rarely gyrose, sometimes elevated in thalline margin parathecium: thin and distinct, 10-20 µm wide, ±pale yellow epihymenium: dark reddish brown, c. 10 µm thick hymenium: ±golden, 80-100 µm tall; paraphyses: 1.5-2 µm wide at base, apices expanded to.3-4 µm subhymenium: pale yellow, 20-30 µm thick; hypothecium: 10-20 µm thick asci: clavate, often scarcely or poorly developed, 50-60 x 10-19 µm, ±100-spored ascospores: hyaline, simple, usually narrowly ellipsoid., 4-5 x 1-1.8(-2.6) µm Pycnidia: not seen Spot tests: UV-, cortex strongly C+ and KC+ red Secondary metabolites: gyrophoric acid (major), lecanoric acid (minor), 3-hydroxygyrophoric acid (trace), methyl lecanorate (trace) (HPLC, J.A. Elix, pers. comm.). Substrate and ecology: on acidic rocks, usually in full sun and generally inland from the coast. The species is vigorously competitive and individual, shiny squamules can often be found among other saxicolous lichens World distribution: North America (western, north to Alaska, Maine) and Russia (Ural Mountains and Novaya Zemlya) Sonoran distribution: Arizona, southern California, and Sonora. Notes: Acarospora thamnina is unmistakable when forming independent, thick-stiped clumps with elevated squamules, but it also grows commonly among other lichens, and then may appear flat, although the occurrence of a stipe may become apparent upon dissection. Acarospora thamnina is often determined as A. fuscata, which is actually uncommon in the Sonoran area. Although A. thamnina occurs in many shades of brown from reddish to yellowish, sometimes on the same rock, it is always shiny and has a strong C+ cortical reaction. The apothecia may be immersed in the squamules or form large conspicuous discs with a thalline margin. They are usually black and almost gyrose in the Sonoran area. But specimens do sometimes have smaller reddish discs, more like A. fuscata. At the same time, rare specimens of A. fuscata on rough surfaces may key out as A. thamnina, but they do not have a true stipe (rather are gomphate); such specimens are often collected in eastern North America, where A. fuscata is the most common Acarospora. The other brown species of the Sonoran area that develops noticeable stipe is A. obnubila, which has a C- thallus and is dull. The type was collected by Bolander in Yosemite National Park at Mono Pass (FH!). Acarospora interposita H. Magn. is a synonym for rare collections of these squamules in Russia. The holotype of A. interposita var. nitidella from the Santa Monica Mountains (UPS!) has very young, dispersed thalli with very small apothecia, and is similar to modern collections made on Sandstone Peak (Ventura County). Acarospora thermophila Herre is a synonym of A. thamnina named from desert specimens (FH!).