Thallus: rimose-areolate; areoles: 0.5-1.0 mm in diam., plane, thin upper surface: white or whitish gray, rough, dull, slightly grayish and whitish pruinose Ascomata: perithecioid, immersed to semi-immersed, up to 2.0 mm in diam.; disc: blackish, plane; proper exciple: up to 70 µm thick; hymenium: 120-140 µm high asci: cylindrical to subclavate, 6 (-8)-spored ascospores: brown, muriform, ellipsoid, 23-29 x 15-19 µm; transverse septa 5-6, longitudinal septa 2-3 per transverse segment Pycnidia: not seen Spot tests: K-, C-, KC-, P-, UV- Secondary metabolites: none detected. Substrate and ecology: on siliceous rocks World distribution: Arabia and western North America Sonoran distribution: rare, at 1600-2200 m in Arizona (and adjacent Colorado and Utah). Notes: Diploschistes arabiensis is characterized by the lack of secondary metabolites and a relatively thick, whitish thallus. A similar species is D. euganeus which differs in having a thinner, more grayish thallus, unridged apothecia, and the predominantly 8-spored asci.