Life habit: lichenized; Thallus: mainly foliose, thin and membrane-like or thick, rarely subcrustaceous, subfruticose or umbilicate; lobes: rounded and sparsely branched to elongated and deeply divided or indistinct; tips: swollen and plicate or even and thin, gelatinous when wet; upper surface: dark olive to black, lobes often with longitudinal ridges and/or pustules, rarely striate, rarely smooth, rarely with white hairs, occasionally pruinose; isidia: present or absent, globose, club-shaped, cylindrical to coralloid or scale-like; lower surface: olive, ±concolorous or paler than upper surface, smooth, striate, pustulate, ridged, sometimes canaliculate, often with tufts of white rhizohyphae; photobionts: primary one a filamentous cyanobacterium with bead-like chains and intercalary heterocysts (Nostoc), secondary photobiont absent; Ascomata: absent or present, apothecial, laminal or marginal, orbicular, immersed, sessile or stipitate; margin: distinct, with persisting, smooth, crenulate, nodulose, isidiate or lobulate, concolorous thalloid rim; disc: dark red, yellowish brown or black; ontogeny: hemiangiocarpous, single ascogonia (with trichogynes) arising from a common thallus hypha beneath the thallus surface; true exciple: cupular, laterally thin or thick, basally and apically often thicker; cells: slender, elongated (euthyplectenchymatous), broad ellipsoid or subglobose (subparaplectenchymatous) or angulate and isodiametric (euparaplectenchymatous), I-; hymenium: hyaline, I+ constantly blue; epithecium: yellowish or reddish brown, K-; paraphyses: straight, simple or apically branched, septa distinct or indistinct; apical cells: usually thickened, clavate to globose, c. 4-7 µm wide; subhymenium: hyaline or pale yellow, I-; asci: narrowly clavate to clavate, Lecanoralean, tholus IKI+ blue with a downward projection and apical cap, 4-8(-16)-spored; ascospores: hyaline, sometimes spirally, narrow or broad fusiform, needle-like, ellipsoid, broad ellipsoid to globose, bacilliform or cubical, rarely flagellate, straight or sinuose, sometimes clavate, 1 to multiply septate with transverse septa only, or muriform with 1-3 longitudinal septa, 10-200 x 2-22 µm; wall: thin; Conidiomata: pycnidial, laminal or marginal, immersed or semi-immersed; ostiolum: usually pale, conidiophores: branched, short celled; conidia: bacilliform, not or ±swollen at both ends, (3-)4-6(-8) x 1-1.5(-1.8) µm, pleurogenous; Secondary metabolites: none detected; Geography: world-wide; Substrate: bark, wood, various rocks, soils, mosses and plant debris.; Notes: Collema is distinguished from Leptogium by the absence of a true cortex. However, the cortex may be poorly developed in certain species of Leptogium (e.g. L. biatorinum) and a pseudocortex may be present in some Collema species (e.g. C. nigrescens) at the lower side of the apothecia. The thallus in most Leptogium species is deep red-brown or bluish or glaucous and often ±shiny or heavily wrinkled. Collema is usually black with a olive tinge, not wrinkled and rarely shiny. Lempholemma has Nostoc photobionts and a similar thallus anatomy to Collema but lacks septate ascospores and amyloid ascus tips. In sterile condition, Lempholemma can be distinguished from Collema by the type of pycnidium and conidiophores. Rock inhabiting species of Collema are often confused with foliose-fruticose members of the Lichinaceae, e.g. Lichinella, Digitothyrea and Thyrea. The latter genera, however, differ in the presence of coccoid photobionts, simple ascospores, types of ascus and pycnidium. Some common species of Collema show considerable variation in morphology and are difficult to identify (e.g. C. coccophorum and C. tenax). Species distinguished on the basis of ascospore septation alone are not identifiable when sterile. Collema confertum Hepp ex. Arnold is probably a Lempholemma species.