Thallus: foliose, medium-sized to large, 2-6(-20) cm, membrane-like, rounded, ±closely adnate, ±deeply and broadly lobed lobes: 0.5-1.5 cm wide, 60-100 µm thick (moist), few, rounded or ±extended, overlapping, often folded; margin: entire and not swollen upper surface: dark olive-green to black, lighter and semi-translucent when moist, dull or somewhat glossy, epruinose, strongly ridged and pustulate; ridges: radiating, short in young parts of thallus, in old parts becoming long and narrow, up to 1.5 mm tall and 0.1-0.3 mm wide, simple or branched, often somewhat flexuous, contiguous or discrete isidia: absent lower surface: paler green than upper surface, at times bluish green, with depressions corresponding to ridges, with scattered, ±rounded, white or dark hapters Apothecia: often ±numerous, dense or rather scattered, especially on ridges, sessile with constricted base, often broadly stipitate, 1-1.5(-2) mm wide disc: at first ±concave, later plane, finally ±convex, dark red or red-brown to pale or dark brown or black, smooth, usually glossy, epruinose or pruinose thalline margin: thin or rather thick, entire, smooth or somewhat wrinkled, not or only slightly prominent, persistent or finally disappearing, usually distinctly pseudocorticate proper exciple: thin to rather thick up to 40 µm wide, euthyplectenchymatous, sometimes subparaplectenchymatous hymenium: hyaline, 85-130 µm tall asci: clavate to narrowly clavate, 8-spored ascospores: hyaline, irregularly clavate, broadly acicular with one end thicker and usually curved or sometimes narrowly fusiform, straight or curved, 5-6(8)-celled, not constricted at septa, (34-)40-75 x (4.5-)6-6.5(-7) µm wide (in K often slightly narrower) Pycnidia: often numerous, laminal or marginal, immersed to semi-immersed, on both sides of thallus, globose or subglobose, pale conidia: straight, bacilliform or with slightly swollen ends, 4.5-6 x 1-1.8µm Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: on ±nutrientrich bark in humid areas, often in open oak woodlands World distribution: North America, Europe, Macaronesia, northern Africa, South Africa, India, and West Samoa Sonoran distribution: mountains of southern California and southern Arizona. Notes: Collema nigrescens is similar. It differs from Col-lema subnigrescens in its sometimes isidiate and often multicolored thallus with pale, yellowish olive depressions and in its slightly narrower, usually acicular ascospores. Collema pulcellum is also similar but its proper exciple is usually distinctly euparaplectenchymatous. Collema subnigrescens f. caesium has distinctly pruinose apothecia, just like C. furfuraceum var. luzonense. The latter species, however, differs in the presence of cylindrical to coralloid isidia. Some varieties of Collema pulcellum also have pruinose apothecia, but are not known to occur in the American Southwest. Collema subnigrescens f. caesium has been distributed from southern California by H.E. Hasse under several names: C. nigrescens, C. nigrescens var. leucopepla (i.e. C. pulcellum var. leucopeplum) and C. glaucophthalmum.