Thallus: foliose, orbicular to irregularly spreading, loosely attached, 5-10(-15) cm in diam., sometimes conspicuously split or torn, flabby, pliable when moist, rather papery and brittle to coriaceous when dry lobes: irregularly laciniate, often deeply lacerate, 8-20(-30) mm wide, 20-70 mm long, complex-imbricate or folded centrally; apices: rounded, discrete to overlapping; margins: delicately scalloped, notched or incised, rarely entire and sinuous, slightly thickened and with a narrow rim below, without cilia upper surface: dark blue-black or glaucous, blue-gray suffused red-brown in parts when moist, pale blue-gray or fawn-colored centrally, marginally often suffused dull charcoal gray to blue-gray, or becoming uniformly dark olive-brown or red-brown when dry, smooth, undulate to slightly wrinkled or ridged, occasionally with patches of crystalline, white pruina (x 10 lens), with minute, white or fawn-colored maculae visible as a faint marbling [best seen at lobe apices (x10 lens)], rarely with minute, styliform isidia, without pseudocyphellae or soredia, phyllidiate phyllidia: often dense, subterete to flattened, coralloid-branched, in conspicuously stalked, palmate clusters, 1-3 mm tall, often crowded at margins or in small, congested groups on the upper surface or developing along cracks or breaks in the cortex medulla: white, K- photobiont: cyanobacterial lower surface: tomentose from margins to center, rarely with a narrow, glabrous marginal zone; tomentum: pale fawn- or cream-colored and rather short at margins, darkening to gray-brown or red-brown and becoming woolly or shaggy centrally, with scattered groups of dulled or felted, long, densely fasciculate rhizines 2-4(-6) mm long developed centrally cyphellae: numerous, scattered, pin-pricklike at margins round to irregular centrally and there rather less frequent, 0.05-1(-1.5) mm in diam., not gaping away from margins, with narrow, flat to slightly raised margins often obscured by tomentum, with a white basal membrane Apothecia: marginal or submarginal, sessile, constricted at base, solitary or in small groups, rounded, 0.5-2(-3) mm in diam. disc: orange-brown to red-brown, concave to plane, becoming distinctly convex at maturity, dull or glossy, epruinose margin: brown-black (darker than disc) above but paler below, coarsely corrugate-scabrid, obscuring disc at first, with occasional minute, silky hairs below; epihymenium: pale yellow-brown, strongly gelified, external to tips of paraphyses, 8-14 µm thick hymenium: hyaline, 85-100 µm tall; hypothecium: pale yellow-brown, 50-70 µm thick asci: clavate, 80-85 x 12.5-18 µm ascospores: hyaline, ellipsoid-fusiform with pointed apices, 33-38 x 5-8.5 µm Pycnidia: occasional to rare, scattered on lamina, visible as pale, fawn or white swellings, 0.1-0.2(-0.4) mm in diam, with a central, depressed, red-brown ostiole conidia: bacilliform, straight, 3-5 x 1 µm Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: on bark of Quercus, and over mossy rocks (rhyolite) and in detritus in pine-oak forest World distribution: apparently endemic to Mexico Sonoran distribution: Baja California, Chihuahua, and Sinaloa. Notes: The Jamaican taxon Stictina quercizans var. dissecta Müll.Arg., [Flora 74: 374 (1891)] is similar, but the lobes of this taxon are much narrower and much more highly dissected than the Mexican material studied. Sticta mexicana is a cyanobacterial species characterized by its large, rather ragged laciniate lobes, its coralloid-branched phyllidia not associated with cilia but rather developing both marginally and laminally in dense, palmate clusters, its tomentose lower surface with short, even and rather velvety tomentum marginally but woolly centrally with entangled or felted clumps of fasciculate rhizines 2-6 mm long, its sessile apothecia with a dull to glossy orange-brown to red-brown discs and with a darker, persistent, corrugate-scabrid margin without projecting cilia, and finally its hyaline, fusiform-ellipsoid ascospores, 1-3-septate, 33-38 x 5-8.5 µm.