Chihuahua: along route 16, c. 28 km W of Basaseachic, riparian area with scattered boulders, on acidic rock, 28º2'N, 108º16'W, 25 July 1994, T.H. Nash III 37834 (ASU - holotype designated here; hb van den Boom, isotype). Thallus: effuse to delimited, rimose to rarely areolate; prothallus: reddish to dark brown areoles: 0.2-1 mm wide, becoming thinner towards the margin, at the center up to 0.5 mm thick, blastidiate at the fissures surface: pale greenish gray or brownish gray, plane, scurfy, epruinose, blastidiate parts paler colored cortex: not differentiated, without an epinecral layer, ecorticate but with a crystal layer up to 70 µm thick; crystals: finely granular (c. 2 µm wide) or clustered (up to 20 µm wide), soluble in K medulla: indiscernible, filled with crystals; algal layer: irregular, filled with crystals; algal cells: rounded, 5-15 µm wide Apothecia: abundant, adnate to sessile, sometimes slightly constricted at base, 0.15-0.6 mm in diam., laminal on the areoles in groups and crowded or scattered, +round or with an irregular flexuose margin disc: chocolate-brown, dark brown to blackish brown (when wet), mostly plane, sometimes slightly convex, epruinose margin: entire, persistent or rarely nearly excluded exciple: paler than the disc, hyaline to grayish or pale brown, up to 50 µm thick in its lower part, laterally ecorticate, lower part paraplectenchymatous, composed of ±isodiametric cells 2-5 in µm diam. with brown walls, K-, without photobiont cells, outer rim filled with fine crystals epihymenium: yellowish to reddish brown, without granules hymenium: hyaline, I+ blue, 80-100 µm tall; paraphyses hypothecium: yellowish brown to reddish brown, with unoriented hyphae, up to 30 µm thick in center including subhymenium asci: cylindrical-clavate to broadly clavate 45-55 x 10-15 µm, with a small amyloid gelatinous outer coat; Byssoloma-type, 8-spored ascospores: hyaline, 3(-4)-septate, clavate, ovoid to oblong-ovoid or ovoid-fusiform, straight, rarely slightly curved, 11-18 x 4.5-6 µm, (1-)3(-4)-septate, often constricted at septa; wall: thin, sometimes smooth, but often halonate; perispore: swelling to 2 µm in K Pycnidia: numerous, immersed in the thallus, c. 0.1 mm in diam., slightly convex, irregular round and pale brown above, hyaline below; conidiogenous cells: elongate, branched, c. 12 x 1.5-2 µm, acrogenous conidia: pyriform, 3.5-4.5x1.5-2 µm Spot tests: thallus K+ yellow, KC-, P- Secondary metabolites: atranorin, and probably divaricatic acid. Substrate and ecology: on acidic rock outcrops, in a riparian area with scattered boulders World and Sonoran distribution: known only from the type locality in western Chihuahua. Notes: Fellhanera nashii may be the first species of the genus obligately occurring on rock, although some species occurring on bark have also been found occasionally on rock. The new species is a typical Fellhanera by its combination of dark brown apothecia with a very small pale margin, a paraplectenchymatous structure of the exciple, halonate ascospores and the pyriform conidia. The thallus is mainly greenish gray and blastidiate. None of the known foliicolous sorediate species is closely related to F. nashii. In habitus, F. nashii resembles F. viridisorediata, a sorediate species on bark from western and central Europe. Although their apothecia appear identical, the ascospores in F. viridisorediata are 1-septate and the pycnidia have never been found in that species (over 50 specimens, in my private herbarium investigated).