1a Filaments over 2 cm long and over 0.1 mm in diameter; photobiont a single-celled green alga
2a Thallus forming black, richly branched, prostrate flat mats on siliceous rock
2b Thallus color various, sparsely branched, prostrate or erect; substrate various but most common on trees
1b Filaments less than 2 cm long or less than 0.1 mm in diameter; photobiont blue-green, often filamentous (LM needed to proceed with confidence from this point)
3a Filaments very fine, main branches < 20 μm (= 0.02 mm) diameter
4a Fungal hyphae on the surface of the filaments very irregular and bent (LM)
4b Fungal hyphae parallel and straight to somewhat knobby, coalescing (LM)
3b Filaments more coarse, main branches generally > 70 μm diameter
5a On rock, mosses over rock, soil, or moss over soil
6a Thallus mat forming; on rock
6b Thallus cushion forming; substrate various
7a Spores brown, 2- to 4-celled; on soil and mosses. Cephalodia fruticose; thallus reduced; arctic-alpine to montane; rare
7b Spores hyaline, spores multicellular to muriform; on all substrates
8a On mosses over rock; spores 2-celled; thallus densely cushion forming, richly branched, with glossy round branches; apothecia usually present
8b On various substrates including moss over rock, spores multicellular to muriform; thallus minutely fruticose or more often with fruticose projections from a dorsiventral thallus
5b On bark
9a Hyaline hairs present on surface (LM); on bark w Cas
9b Hyaline hairs lacking; substrate various
10a Cortical cells in jigsaw pattern (LM); photobiont Scytonema (cylindrical filamentous)
10b Cortical cells forming a mosaic but not jigsaw patterned
11a Photobiont Nostoc (beaded-filamentous)
11b Photobiont Scytonema (cylindrical-filamentous)