Thallus: umbilicate, monophyllous, +somewhat lobate, +circular, brittle, thin, rigid, usually with an entire margin, 1-3(-5) cm in diam. upper surface: light to medium brown, shiny, smooth but with irregular, thin, black cracks in the surface giving a chinky appearance, usually epruinose medulla: white lower surface: light gray to light brown, sometimes black around the umbilicus, coarsely papillate Apothecia: common, black, immersed to sessile, circular or angular, with gyrose disc often with many small circular elevations, up to 2.5 mm in diam. ascospores: hyaline to light brown, simple, ellipsoid, 8-16 x 4-9 µm Spot tests: K-, KC+ red, C+ red, P- Secondary metabolites: gyrophoric, lecanoric and umbilicaric acids. Substrate and ecology: on siliceous boulders in very xeric microhabitats World distribution: western North and South America Sonoran distribution: occurring in many localities, from near the seashore to high mountains in Arizona, southern California and Baja California. Notes: Umbilicaria phaea is the most widespread rock tripe in the area, something that reflects its ability to grow in very dry habitats. A disjunct specimen of U. phaea var. coccinea Llano is reported for northern Baja California (Llano 1950), although this distinctive red-colored taxon primarily occurs in the Pacific Northwest. The common name of U. phaea is emery rock tripe.