Thallus: squamulose, indeterminate, up to 4 cm across squamules: variously shaped, with a stipe, up to 3 mm wide, up to 1.5 mm. thick; contiguous and crowded; rim: ±down-turned upper surface: yellow, dull, convex, smooth to rough, with shallow fissures, epruinose upper cortex: anticlinal prosoplectenchyma with angular to round lumina, 60-130 µm thick, syncortex: indistinct; eucortex: upper one-fourth yellow, hyaline below algal layer: thin, uneven above and below, penetrated by hyphal bands (best observed in thick section at low magnification) medulla: white, prosoplectenchymatous lower surface: black, corticate, thin attachment: developing a black stipe, less than half of diam. of squamule Apothecia: 1-12 per squamule disc: yellow- to red-brown, immersed, 0.1-0.5 mm across, often punctiform or slit-like parathecium: 10-50 µm in width epihymenium: usually greenish yellow, 20-25 µm thick hymenium: hyaline, 130-170 µm tall; paraphyses: 2-2.5 µm wide at base, with barely expanded apices subhymenium: pale gray to hyaline, 30-60 µm thick; hypothecium: indistinct, up to 20 µm thick asci: narrowly clavate, 110-130 x 20-24 µm, 100+ spored ascospores: hyaline, simple, mostly broadly ellipsoid, 4-6 x 2-2.5(-3) µm Pycnidia: c. 200 µm in diam. conidia: bacilliform, c. 2 x c. 1 µm Spot tests: UV+ orange, spot tests all negative Secondary metabolite: rhizocarpic acid. Ecology and substrate: on volcanic or acid rock World distribution: southwestern North America, Mexico, South America (Argentina, Venezuela ) Sonoran distribution: Arizona (Cochise County, Chiricahua Mountains, 1890-2700 m). Notes: The distinctive feature in A. brouardii is its black, corticate underside and narrow stipe. The squamules can get quite large, but young squamules, possibly initially parasitic on Caloplaca, are bright yellow, rather thin and narrow. The description will need revision when more specimens are available. It may basically have a Mexican distribution; and it may be overlooked and undercollected elsewhere in the American Southwest. Some specimens of A. socialis have black undersides caused by fungal infection, and hence resemble A. brouardii in this feature.