Thallus: foliose to weakly subfruticose, up to 2.5 cm (rarely to 5 cm) in diam., +orbicular to rather irregular lobes: short and rounded to more often elongate and much branched and laciniate, 0.3-1 mm broad, flat to weakly concave, prostrate or frequently ascending upper surface: tan-green to olive-green or olive-brown, sometimes almost black in old central parts, epruinose, dull or weakly shiny at the periphery, the lobe edges (and occasionally the older lamina) sometimes warty-papillate, the papillae +pseudocyphellate on the end, without soredia or isidia upper cortex: paraplectenchymatous medulla: white lower cortex: paraplectenchymatous lower surface: almost white to pale tan, smooth to weakly rugose, +dull; rhizines: sparse, simple, concolorous with the lower surface or darkening Apothecia: frequent, up to 4.5 mm in diam.; margin: becoming crenate to foliolate, bearing pseudocyphellae on the crenae or edges of the folioles (rarely with rhizines developed on the folioles) ascospores: ellipsoid, 7.5-9.5 x 4-5 µm Pycnidia: black, weakly to moderately emergent (especially those on marginal laciniae appearing strongly emergent), mostly marginal or submarginal conidia: bifusiform, 5-6 x 1 µm Spot tests: cortex and medulla all spot tests negative Secondary metabolites: 2-3 high Rf, lichesterinic acid type fatty acids. Substrate and ecology: usually bark or wood of conifers World distribution: North America Sonoran distribution: throughout southern Rocky Mountains of Arizona and into Chihuahua State of Mexico. Notes: Tuckermanella fendleri, even in the strict sense defined here, is still a highly variable species. Variation in lobe conformation, from narrow and linear-elongate to rather irregularly rounded-imbricate, in habit from prostrate to loose or somewhat ascending (usually only in parts) and in color from blackish green to pale olive- green or olive-brown can be confusing to say the least. The largest, broadest lobed specimens, for instance, might be misdetermined as Tuckermannopsis orbata. The color range in T. fendleri is very similar to that of Kaernefeltia merrillii, and the uncommon partly prostrate specimens of the latter species might be confused with T. fendleri. The upright portions and even the more prostrate thallus segments of K. merrillii can be distinguished by the near absence of rhizines and the darker (usually brown to dark brown) and more rugose or pitted lower surface. When fertile, the dark green-brown to green-black apothecial discs of K. merrillii are clearly different from the brown or chestnut discs of T. fendleri.