Thallus: crustose, areolate to granulose, spreading areoles: irregularly shaped, 0.25-0.75(-1) mm wide, 0.3(-0.5) mm thick, ±contiguous upper surface: black, rough, warty or slightly granulose, dull; isidia: absent lower surface: attached by rhizohyphae Apothecia: semi-immersed to sessile, (1)2-4(5) per areole, small, (0.1-)0.2-0.4(-0.6) mm wide disc: at first punctiform, later open and somewhat depressed, black when dry, dark red when moist, with persisting, smooth thalline margin proper exciple: lacking or very thin, up to 10 µm wide epihymenium: yellowish brown, K- hymenium: hyaline, (85-)100-110(-120) µm tall, IKI+ blue asci: subclavate, 8-spored ascospores: simple, hyaline, broad ellipsoid, (10-)12.5-17.5(-20) x 7.5-10 µm; wall: thin Pycnidia: not observed conidia: not observed Spot tests: all negative Secondary products: none detected. Substrate and ecology: on mostly calcareous rocks including man-made substrates; usually in exposed habitats World distribution: North America, Europe, and Arabia Sonoran distribution: Arizona, Sonora, and Baja California Sur. Notes: Psorotichia murorum is characterized by its small, irregularly shaped, lightly granulose areoles with 2-5 apothecia per areole. The apothecia are small and have a depressed disc, and the proper exciple is very thin or lacking. In contrast, Psorotichia schaereri usually has slightly larger, distinctly granulose areoles with 1-2 apothecia per areole. The apothecia are slightly wider and usually have a plane, open disc, the proper exciple is thin but usually visible as a pale ring between thalline margin and apothecial disc.