Thallus: placodioid, effigurate and determinate, overall up to 6 cm wide areoles: orbicular to irregular, 1-2 mm wide, flat but becoming convex and up to 1 mm. thick, becoming confluent, usually with a distinct margin thickened peripherally up to 0.4- 0.5 mm thick; lobes: 0.5-2 mm long, 0.6-1.3 mm wide, poorly subdivided; interior areoles: 0.4-1.3 mm wide, 0.3-0.6 mm thick, occasionally becoming subsquamulose, crowded and imbricate, pushing upward; rim: widened and revolute upper surface: yellow, dull, plane to convex, rough, uneven; center appearing slightly black-tinged in some specimens, epruinose algal layer: 60-300 µm thick, even and uninterrupted medulla: white, prosoplectenchymatous, 100-110 µm thick, indistinguishable from attaching hyphae attachment: broad, sometimes becoming somewhat thickened, but without a stipe Apothecia: usually one, immersed in central areoles, sometimes becoming elevated and lecanorine with fissured thalline margin, 0.2-0.5 mm across disc: reddish and rough epihymenium: diffusely yellow, c. 30 µm thick hymenium: hyaline, 80-130 µm tall; paraphyses: 1.7-2.1 µm wide at base subhymenium: pale yellow to hyaline, 30-50 µm thick; hypothecium:10-15 µm thick asci: clavate, 60-80 x 20-23 µm, 100+-spored ascospores: hyaline, simple, ellipsoid, 3-5 x 1.8-2 µm Pycnidia: 1-4 per areole 150-200 µm in diam. conidia: bacilliform, 2-2.5 x 1 µm Spot tests: UV+ orange, all spot tests negative Secondary metabolites: rhizocarpic acid (major), epanorin (trace), conrhizocarpic acid (trace), vulpinic acid (trace) (HPLC analysis of Nash #12641: J.A. Elix, pers. comm.) Substrate and ecology: on granite in sun World distribution: southwestern North America (New Mexico, Missouri, Oklahoma) Sonoran distribution: Arizona, southern California, Baja California Sur, and Sonora.. Notes: Though not found in Arizona yet, A. novomexicana is expected. It is somewhat similar to A. hilaris, but the lobes of A. novomexicana are apparently wider and thicker, and overall its thallus becomes much thicker. Acarospora novomexicana needs further study as few specimens were seen. Both A. extenuata H. Magn. and A. texana H. Magn. are probably synonyms and either name would have priority. The type of A. extenuata was in herbarium of Bouly de Lesdain and is presumed lost in the bombing of Dunkirk. The other specimen of A. extenuata (Magnusson 1929a) from Argentina is probably A. boliviana. The species also needs a fuller study in relation to a revision of A. hilaris.