Thallus: foliose, medium-sized to rarely large, 1-5(-10) cm broad, adnate, often stunted and convoluted lobes: usually 0.5-1 cm wide, (60-)90-150 µm thick, apically rotund, tips often somewhat ascending upper surface: densely pustulate and coarsely ridged, ridges usually low; dark olive-green to brown or rarely black, in old parts between pustules and ridges very often yellow to pale yellowish brown, dull or somewhat glossy, epruinose isidia: present or not, laminal and marginal, sometimes abundant, globular to slightly oblong or flattened, up to 0.2 mm wide lower surface: greenish brown, pitted Apothecia: common, usually numerous and crowded, often covering most of the thallus, (0.4-)0.6-1(-2) mm wide disc: plane or convex, brownish red, usually glossy, smooth, epruinose thalline margin: entire, often isidiate, pseudocorticate proper exciple: thin, euthyplectenchymatous hymenium: hyaline, 85-110 µm tall asci: clavate, 8-spored ascospores: hyaline, acicular to bacilliform, straight or spirally curved, transversely septate, (5-)6-13-celled, 50-90(-112) x 3-4.5 µm Pycnidia: laminal or marginal, immersed or semi-immersed, on both sides of the thallus, pale conidia: bacilliform or with slightly swollen ends, 4.5-6 x 1-1.8 µm Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: on acid, or ±weakly nutrient-rich bark in riparian or coastal situations, occasionally on rocks World distribution: from the Arctic to Mediterranean areas of Europe, Asia, and North America, extending to northern Africa, the Near East, and Oceania Sonoran distribution: restricted to southern California and Channel Islands. Notes: For separation of Collema nigrescens from Col-lema furfuraceum see under the latter species. Collema subnigrescens is similar in many ways, but lacks any isidia and usually has slightly broader and somewhat shorter, distinctly clavate ascospores. Collema pulcellum has a similarly pustulate and ridged thallus, and has several varieties in the eastern, southern and northern U.S.A. It is clearly distinguished by its euparaplectenchymatous proper exciple. Among the southern Californian samples of Collema nigrescens distributed by H.E. Hasse those lacking isidia and having distinctly pruinose apothecia belong to C. subnigrescens f. caesium (see there); those with cylindrical isidia and pruinose discs belong to C. furfuraceum var. luzonense.