Thallus: usually effigurate, areolate to rimose-areolate centrally, determinate, orbicular and 1-5 cm wide or often irregularly shaped areoles: mostly angular, plane to convex, 0.4-1 mm in diam., 0.4-1 mm thick (usually appearing very thin), confluent and losing orbicular shape; marginal lobes: elongate, contiguous, apices expanded and blunt upper surface: yellow, glossy similar to Pleopsidium, plane to convex, rough, uneven, with fissures eventually becomingdivision lines on areoles, epruinose cortex: paraplectenchymatous, pale yellow throughout with obscure cells, 30-50 Ám thick algal layer: even, dense medulla: white, intricately prosoplectenchymatous attachment: broad, without forming a stipe Apothecia: numerous in central areoles; occasional in outer areoles disc: yellow, sometimes with a black-tinge, immersed, punctiform, rarely up to 0.6 mm wide, usually much smaller epihymenium: dark or light yellow, conglutinated, 15-25 Ám thick hymenium: hyaline to pale yellow above, 100-170 Ám tall; paraphyses: 1.5-2 Ám wide at base, apices barely expanded subhymenium: hyaline, 20-50 Ám thick; hypothecium: 10-15 Ám thick asci: clavate, 60-100 x 20-25 Ám, 100+-spored ascospores: hyaline, simple, ellipsoid to narrowly ellipsoid, 4-6 x 1.7-2.2 Ám, but sometimes varying in size Pycnidia: not observed Spot tests: UV+ orange, C+ red, KC+ red in cortex, medulla, and apothecia Secondary metabolites: rhizocarpic acid (minor), epanorin, gyrophoric acid (major), lecanoric acid (minor) (HPLC: J. A. Elix, pers comm.) World distribution: Mexico, southwestern United States Sonoran distribution: Arizona, Baja California Sur, Chihuahua, Sinaloa, and Sonora. Notes: Acarospora erythrophora is usually effigurate and has been mistaken for a Pleopsidium. The C+ red/KC+ red reactions are distinctive. Acarospora robiniae also occurs in Baja California but has only a C+ red cortex, a thallus generally composed of dispersed areoles with yellow discs, and a strictly maritime distribution. The type locality of A. erythrophora is in New Mexico, and it occurs inland in Arizona and Sonora.