Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2004. Lichen Flora of the Greater Sonoran Desert Region. Vol 2.
Editorial comment (F. Bungartz): At the time of publication of the Greater Sonoran Desert Flora, Umbilicaria semitensis was considered a synonym, but McCune & Curtis (2012, The Bryologist115(2): 255-264) demonstrated the two taxa to be distinct species The photo insert of Vol. III of the Sonoran Flora is erroneously identified; the immersed apothecia suggest that the image belongs to Umbilicaria phaea.
Thallus: umbilicate, monophyllous, +circular, thick, rigid, often folded, 1-4(-9) cm in diam.; upper surface: light to dark brown to grayish olive, smooth to obscurely wrinkled to sometimes cracked, dull to somewhat shiny, marginally pruinose; lower surface: dark brown to black, coarsely papillate where not densely covered with ivory white slender, branched rhizinomorphs; medulla: white. Apothecia: common, black, sessile, angular to star-shaped, gyrose, up to 2 mm in diam.; asci: clavate, 8-spored; ascopores: hyaline, simple, ellipsoid to broadly ellipsoid, 12-20(-29) x 7-15(-18) µm. Spot tests: medulla K-, C+ red, KC+ red, P-. Secondary metabolites: gyrophoric, lecanoric, and hiascic acids.
Substrate and ecology: on dry, exposed, siliceous rocks; World distribution: the west coast of North America, scattered from California to Alaska; Sonoran distribution: scattered in the coastal ranges of southern California.
Notes: Umbilicaria angulata, a species endemic to the west coast of North America, is very similar to U. phaea. The major difference is the rich development of rhizinomorphs on the lower surface of U. angulata. Umbilicaria phaea also seems to have a tolerance for even drier, inland habitats. According to Llano (1950) the spores may become muriform in U. angulata.
Description.Thallus mid brown to blackish brown, sometimes with faint violet tones, matte to slightly shiny, to 7 cm diam; upper surface smooth to weakly areolate; lower surface dark brown to black but lacking thalloconidia, verrucose, usually developing a dense mat of tangled black cylindrical to flattened rhizines, sometimes with trabeculae, though parts lacking rhizines and trabeculae are common and some nearly erhizinate individuals can be found; apothecia black, initially level with the thallus but protruding with age, American football shaped to angular or stellate, occasionally roundish; spores simple, hyaline, 8 per ascus, 17–20(22.5) 3 10–13 mm.
Secondary chemistry. Not investigated, but reportedly containing gyrophoric acid and related compounds.
Distribution. Alaska south to northern California, not known east of the Cascade Crest in the Columbia Basin (Fig. 4; report in DeBolt & McCune (1993) from Montana misidentification of unusual forms of U. torrefacta with blackish, rhizinate lower surface).