Collema furfuraceum (Arnold) Du Rietz forme corticole.
Thallus foliose, unfolded, rosette-shaped, 5-10 cm diam., conspicuously lobed, lobes 1-1.5 cm wide, thin and membrane-like even when moist ('bat's wing' -like), tips more or less recurved, upper surface pleated, markedly ridged, ridges more or less pustulous, dark green-brown, green-brown, dark green-olive when moist, blackish when dry, covered from ridges with cylindrical or coralloid isidia and possibly overlaying most of the surface in old specimen, producing a dark green-olive velvet-like aspect. Apothecia very rare, 0.5-1.5 mm diam., disc flat, with an isidiate thalline exciple. Ascospores fusiform to acicular, often curved, 4- to 5-septate, 40-80 x 3-7 µm. Chemical spot tests negative. Photobiont : Nostoc. This species has an oceanic trend and grows on mossy bark and horizontal boughs and occasionally on mossy rocks in protected areas near the sea shore, see: Collema furfuraceum forme saxicole. N.B. Collema nigrescens has globular isidia and Collema subflaccidum has neither wrinkles nor pustules.*