Thallus: loosely adnate to adnate; appressed throughout or somewhat reflexed at the periphery, foliose, up to 7 (10) cm diam., lobate lobes: short and rounded to somewhat elongate, contiguous to imbricate, l-3 (-5) mm broad, ± flat upper surface: pale olive-green to olive-brown, reddish brown, dark brown or blackening, smooth to weakly and irregularly pitted and/or wrinkled, rarely becoming lobulate in the center; dull to more often strongly shiny, especially near the periphery; without pseudocyphellae isidia: sparse to dense, ± cylindrical, often branched, attenuate or distinctly knobbed at the end, up to 0.8 (-1.5) mm long and 0.05-0.1 (-0.5) mm diam., easily and frequently broken off near the base and leaving conspicuous white spots lower surface: dark brown to black, somewhat paler near the periphery, occasionally almost smooth but more often rather evenly rugulose, usually distinctly shiny, occasionally dull; moderately rhizinate, the rhizines ± concolorous with the lower surface Apothecia: occasional, up to 6 mm diam., sessile, concave to flattening or somewhat irregular, margin entire when small, soon papillate and/or isidiate, easily and commonly eroded asci: clavate, 8-spored ascospores: ellipsoid to rarely almost subglobose, 10-14 x 5.5-9.5 µm Pycnidia: rare, immersed conidia: weakly bifusiform, 6-7.5 x 1 um Spot tests: cortex K-, C-, KC-, P-, HNO3-; medulla K- (or K+ violet in occasional pigmented parts), C+ red, P- Secondary metabolites: lecanoric acid, unknown TE-12 (minor) (Esslinger 1977), skyrin (minor, in the lower part of the medulla). Substrate: rocks or bark, occasionally wood World distribution: western and eastern North America, south in the mountains, Europe Sonoran distribution: on bark in oak forests in California and on rock in montane conifer forests in Arizona. Notes: This species, with its true isidia and C+ red medulla, is most commonly confused with M. subaurifera, a similar species distinguished by much finer, usually unbranched isidia which are accompanied by soredia, and having a much less shiny upper cortex. Poorly developed or otherwise questionable specimens can be distinguished chemically, because M. subaurifera lacks unknown TE-12 (Esslinger 1977), which is uniformly present in M. fuliginosa. The skyrin often found in the lower part of the medulla in this species is also missing from M. subaurifera. An even rarer species that might be confused with M. fuliginosa is M. villosella.