Thallus: determinate, effigurate to rimose areolate, orbicular or not, often confluent, up to 6 cm or more across, areoles: irregularly shaped, 0.5-1 mm across, mostly fertile, mostly densely contiguous and distinctly radiating, margin: with lobes 1.5-2.3 mm. long and as wide upper surface: bright yellow, shiny, rough and becoming rugose upper cortex: 40-70 µm thick, mostly yellow algal layer: 40-90 µm thick, continuous or becoming irregularly interrupted by hyphal bundles, with an uneven surface medulla: intricate to anticlinal prosoplectenchyma with granules, thickness varying, without a lower surface, continuous with attaching hyphae Apothecia: less than 1 mm in diam. disc: usually yellow, plane to very convex; margin: never completely excluded true exciple: parathecium: often forming parathecial crown, up to 50 µm thick, epihymenium: dark greenish yellow, 20 µm thick hymenium: mostly dirty or pale yellow, 65-100 µm tall; paraphyses: conglutinate, with many septae, 1.7-2 µm thick at base; tips: not swollen subhymenium: hyaline, 50-80(-170) µm thick, with mainly anticlinal hyphae; hypothecium: hyaline, 30-50 µm thick, the hyphae parallel or intricate asci: clavate, 50-70 x 16-24 µm, 100+-spored ascospores: hyaline, rather narrowly ellipsoid, simple, 4-5 x 1.7-2 µm Pycnidia: deeply immersed with pale ostioles conidia: 2-2.5 x 0.7-0.8 µm Spot tests: thallus UV+ orange, K-, C-, KC-. P- Secondary metabolites: containing rhizocarpic, acaranoic and acarenoic or lichesterinic acid . Substrate and ecology: on perpendicular or overhanging hard acid rock surfaces, usually middle-level montane to alpine World distribution: Europe and North America Sonoran distribution: Arizona, southern California, Baja California, Baja California Sur, Chihuahua, and Sonora. Notes: Variation in the species is wider than most descriptions suggest, leading to some confusion. Sometimes all or part of the thallus is smooth, sometimes the margin becomes much reduced and the disc can become very convex and appear biatorine but is always less than 1 mm wide.
Thompson, J., 1997. American Arctic Lichens: The Microlichens.
Thallus forming large circular areas, the margins lobate-radiate with lobes to 2 mm long, 0.5 mm broad, slightly convex or flat, often scabrid-surfaced, branched and incised, the center verrucose-areolate, bright yellow or greenish yellow; underside pale.
Apothecia solitary in the areolae, immersed; disk to 0.8 mm broad, dark reddish yellow, concave to flat, with margin thick and obvious, becoming crenulate; exciple dark yellowish, 30-50 µm; hypothecium indistinct, I+blue; epihymenium greenish yellow; hymenium 65-90 µm, yellowish, striate, I+blue; para-physes separating easily, ca. 2 µm, not capitate; spores numerous, narrowly ellipsoid, 4-5 x 1.5-2 µm.
This specics grows on granitic rock outcrops, cspccially under overhangs. This is a broadly distributed arctic to alpine species, common in liuropc. less so in North America, where it ranges south to California and Mexico.
This species is very close to A. chlorophatm in appearance, and Weber (1968) considered them conspccific.