Thallus: foliose, 2-6 cm in diam., adnate, irregularly lobate lobes: very irregular, thickened (swelling markedly when wet), often anatomosing, 2-4 mm wide, 150-800 µm thick; apices: rotund, thickened, entire or isidiate upper surface: usually olivaceous brown to brownish black, dull, +granular, heavily wrinkled longitudinally isidia: scattered to very dense, usually laminal but sometimes marginal, spherical or granular to subcylindrical, simple, concolorous with the thallus or darker internal anatomy: with upper and lower cortices consisting of a single layer of irregularly isodiametrical cells 4-7 µm in diam., internally with loosely interwoven chains of Nostoc and hyphae lower surface: pale to medium gray, wrinkled, with scattered tufts of white hairs Apothecia: frequent, laminal, sessile, on thickened lobe margins, 0.5-2 mm wide disc: light brown to red-brown, concave to plane margin: thalline, concolorous with the thallus, wrinkled, granularly isidiate exciple: euparaplectenchymatous, 45-90 µm thick centrally hymenium: hyaline below and thinly yellow or brown above, 90-135 µm tall; paraphyses: unbranched, 1-2 µm wide, slightly inflated apically; subhymenium: yellow to pale brown, 35-70 µm thick asci: cylindrico-clavate, 8-spored ascospores: hyaline, submuriform, 3-5-septate transversely, 0-1-septate longitudinally, ellipsoid to subfusiform, 20-35 x 9-12 µm Pycnidia: not observed Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: common on bark of soft-barked oaks at intermediate elevations in montane forests World and Sonoran distribution: relatively common in southeastern Arizona and adjacent parts of Sonora and Chihuahua, and Baja California Sur. Notes: Leptogium milligranum looks rather like a Col-lema and has a rather poorly defined cortex, but Collema species in the region have such extensively anastomosing lobes. Anatomical sections are necessary for confirmation. Due to its similar morphology L. milligranum may be the isidiate counterpart of the L. chloromelum. The former species is a common component of semi-arid oak woodlands whereas the latter species extends from the tropics into warm, temperate areas in southeastern U.S.A. Reports of L. milligranum from Ontario and India are apparently misidentifications.