Thallus: rimose-areolate; areoles: 0.5-1.5 mm in diam., plane, thin upper surface: whitish gray to gray, smooth, shiny or dull, epruinose Ascomata: perithecioid, immersed, up to 3 mm in diam.; disc: blackish, plane; proper exciple: up to 70 µm thick; hymenium: 120-160 µm high asci: cylindrical to subclavate, 4-8-spored ascospores: brown, muriform, ellipsoid, 16-32 x 10-20 µm; transverse septa 4-6, longitudinal septa 1-3 per transverse segment Pycnidia: immersed conidia: bacilliform, 4-7 x 1.0 µm Spot tests: K-, C+ red, KC-, P-, UV- Secondary metabolites: lecanoric acid (major), diploschistesic and orsellinic acids (both minor). Substrate and ecology: on siliceous rocks World distribution: subcosmopolitan in relatively open habitats in arid, semi-arid and Mediterranean areas Sonoran distribution: scattered in mountains at c. 500- 2400 m in Arizona, Chihuahua, and Sonora, and about sea level at the coast in Baja California and California. Notes: Diploschistes actinostomus is characterized by the perithecioid ascomata, the presence of lecanoric acid and the whitish gray to gray thallus. A similar species is D. caesioplumbeus which differs in having larger ascospores and darker thallus. Poorly developed thalli of D. actinostomus may be similar to D. euganeus which is readily distinguished by the absence of secondary metabolites and the broadly ellipsoid ascospores.