Thallus: dwarf-fruticose, forming minute crust-like cushion, 1-2 cm wide lobes: terete to somewhat flattened, 0.1-0.2 mm wide, 0.3-1 mm long, up to 200 µm thick; apices: developing long cylindrical to coralloid isidial-like appendages surface: olivaceous brown to brown, smooth, usually not isidiate internal anatomy: with upper and lower cortices consisting of irregularly isodiametrical cells 5-9 µm in diam., internally paraplectenchymatous and with loosely interwoven chains of Nostoc Apothecia: very rare, terminal, stipitate, 0.2-0.4 mm wide disc: red-brown, concave to plane margin: thalline, concolorous with the thallus, entire or isidiate exciple: lacking hymenium: hyaline below and thinly yellow to brown above, 90-100 µm tall; paraphyses: unbranched, c. 1.5 µm wide, slightly inflated apically; subhymenium: pale gray, 25-35 µm thick asci: cylindrico-clavate, 8-spored ascospores: hyaline, submuriform to muriform, (1-)3-5-septate transversely, 1-septate longitudinally, ellipsoid to subfusiform, 20-25 x 10-11 µm Pycnidia: not observed Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: on basic bark, moist rocks or occasionally among mosses on the ground. World distribution: widely scattered in temperate regions of the Northern Hemisphere in western Europe and North America Sonoran distribution: Arizona and southern California. Notes: Leptogium teretiusculum is a quite variable species, that sometimes forms rather flat, expanding thalli, particularly on wet rocks (= L. microphyllum Nyl.), but in other cases forms dense isidioid cushions (on bark) and among mosses on the ground. The taxonomic value of these different forms are in need of further investigation. A particularly deviant form is known from rocks in the Channel Islands. It has shiny blackish isidia.