Thallus: areolate, overall to 5 cm wide areoles: round to angular, up to 1.3 mm in diam., up to 1 mm thick, contiguous or dispersed; rim: ±down-turned upper surface: waxy yellow, flat to convex, glossy, smooth to lumpy, epruinose cortex: prosoplectenchymatous to paraplectenchymatous, with hyphae having irregular, cellular lumina, 80-100 µm thick; syncortex: less than 10 µm thick; eucortex: upper layer: yellow, obscure in water, 40-50 µm thick; lower layer: hyaline, 30-40 µm thick algal layer: 70-90 µm thick, becoming thin and jagged, whitish yellow with penetrating hyphae medulla: white, prosoplectenchymatous, continuous with attaching hyphae lower surface: white or stained by minerals, ecorticate attachment: broad, only occasionally forming a stipe on uneven substrates Apothecia: punctiform to round, immersed, one or more per areole disc: yellow, lighter than thallus, smooth and plane, sometimes with interascal plectenchyma parathecium: indistinct epihymenium: strongly yellow, conglutinated, 15-30 µm thick hymenium: pale yellow above, hyaline below, 80-120 µm tall; paraphyses: c. 2 µm wide at base, apices conglutinated, barely expanded subhymenium: hyaline, 20-30 µm thick; hypothecium: indistinct asci: cylindrical, height varying with the hymenium, 100+-spored ascospores: hyaline, simple, ellipsoid, 3.8-4 x 1.7-2 µm Pycnidia: globose, 80-100 µm wide conidia: ellipsoid, 2-4.3 x 1.3-2 µm Spot test: UV+ orange; C+ red cortex Secondary metabolites: ovoic acid (major), acaranoic acid (major), gyrophoric acid (minor), rhizocarpic acid (minor) (HPLC, J.A. Elix, pers. comm.). Substrate and ecology: usually on volcanic rocks near the ocean in maritime habitats, often competitive with Dimelaena radiata and possibly parasitic upon it Sonoran distribution: southern California (with northern limit in Santa Barbara County) and Baja California Sur. Notes: Collectors have overlooked A. robiniae because C and KC tests are not usually applied to yellow Acarospora. It occurs within the range of the polymorphic and dominant A. socialis, and they are mixed at some sites. Besides the occurrence of gyrophoric acid in the cortex, A. robiniae differs from A. socialis in having a broad attachment of its areoles, a brighter yellow color, yellow discs, and a strictly maritime distribution.