Thallus brownish gray to olivaceous buff at periphery, darker brown with orange tinge centrally, occasionally blackish, greenish when wetted, squamulose with imbricating repeatedly subdivided squamules, central ones 2–5 mm wide, producing ascending to erect lobules, forming rather compact mats or cushions to 6(–8) cm, with robust marginal lobules, 0.2–0.4 mm thick, these with crenate margins that are sometimes slightly white-pruinose, looking rather like cat-paws. Upper surface smooth or weakly cracked areolate, glossy, often brownish when dry, becoming greenish when wetted (hence the epithet). Lower surface white to pale buff, blackish towards center, cotton-woolly with fascicles of brownish black rhizohyphae, producing up to 5 mm thick mat. Thallus 200–400 µm thick with small-celled (6–10 µm diam.) sclerenchymatous upper cortex; photobiont green algae (Myrmecia?) consisting of globose cells, 6–10 µm diam. Apothecia rare, 2–3(–5) mm diam. with lobulate thalline margin and excluded proper margin; disc convex, sometimes proliferating, brown, occasionally blackish. Hymenium hemiamyloid, to 100 µm; asci subclavate, 70–85 X 20–25 µm, with amyloid tubes, 8-spored; ascospores colorless, simple with oil droplets, broadly ellipsoid,(10–)14–18(–20) X (7–)8–11(–12) µm. Pycnidia not observed.

Chemistry. All reactions negative, no lichen acids detected by TLC.

Remarks. This is the first species of Fuscopannaria known to contain green algae, although otherwise with typical characters of the genus. As pointed out previously (Jørgensen 1994), the type of photobiont is not a good taxonomic character, at least at the generic rank. Fuscopannaria viridescens is distinct from F. praetermissa (for its characters see Jørgensen 2001: 687) also in other characters than that of the photosynthetic partner–the morphology is dissimilar, the spores are clearly smaller, and the chemistry is different.  This new species may be mistaken for basal squamules of Cladonia or other squamulose tundra lichens with green algae, especially when sterile, as it often is. It is, however, readily distinguished from these (e.g., species of Catapyrenium, Phaeorrhiza, and Psora) by the thick caespitose squamules with ascending lobules and the well developed black rhizohyphae.  When fertile, it may be mistaken for Psoroma hypnorum, which has larger concave apothecia with small-squamulose margins and cephalodia.  In the sterile state it is best distinguished from sterile thalli of P. hypnorum by the thicker, browner squamules with sclerenchymatous cortex. 

Distribution and habitat. The species is known from coastal Arctic and Subarctic (mountain tundra belt) regions of Siberia and Alaska, mainly on mesic tundra soil or on moribund association bryophytes in lichen-moss dwarf shrub in association with Arctous, Betula nana, Cassiope, Dryas, Empetrum, and Salix spp. It is also present in dry polygon tundra. It is infrequent, but locally abundant and covers considerable areas. Since this type of vegetation has a more continuous and wider distribution than F. viridescens, we presume that this lichen may prove to be more common than presently known, particularly in the region between the type locality and the occurrence on the Lena River delta (where it is quite common, but always sterile), perhaps occurring even further westwards along the northern coast of Russia. It may prove to be one of Thomson’s (1972) Beringian radiants.