Thallus: foliose, up to 4.5 cm in diam. but often smaller, ± regular and orbicular lobes: rather elongate and linear, discrete to contiguous or somewhat imbricate, 0.5-1.5 mm broad, ± flat to irregularly concave, prostrate upper surface: gray to gray-brown or dark brown, usually pruinose over much of the upper surface, sorediate soredia: in marginal and terminal soralia on short side branches, the marginal ones often axillary, discontinuous to occasionally almost continuous, forming by separation of the upper and lower cortex and often becoming ear-shaped or hooded (reminiscent of the "nest-shaped" soralia of Xanthoria fallax), terminal soralia formed similarly, but often appearing reflexed-labriform; soredia granular, greenish to brownish or sometimes noticeably yellowish upper cortex: paraplectenchymatous medulla: white (areas close to the soralia may be pale yellowish) or occasionally discolored lower cortex: irregularly prosoplectenchymatous, in part poorly delimited from the medulla lower surface: peripheral lobes usually whitish to pale tan for some distance (up to 3 to 4 mm in some cases) from the tip, inward becoming dark brown or black, dull to slightly shiny; rhizines: black and squarrosely branched Apothecia: infrequent, up to 2 mm in diam., the margin thick and becoming lobulate, the lobules eventually developing reflexed soralia on the ends and often becoming quite long (sometimes becoming longer than the breadth of the apothecium) ascospores: 33-38 x 15.5-18 µm Spot tests: cortex all negative; medulla K-, C-, KC- (positive K or KC tests may be obtained if tests are done too close to the soralia or on a lobe edge where unnoticed incipient soralia may occur), P-; soralia K+ faint to dark yellow, C-, KC+ yellow or yellow-orange, P- Secondary metabolites: secalonic acid A (apparently restricted to the soralia). Substrate and ecology: bark or (less often) rock World distribution: western North America: Washington, California and Baja California (Guadalupe Island) Sonoran distribution: southern California and Guadalupe Island (Baja California). Notes: With the marginal, sometimes conspicuously pigmented soralia, this species is most likely to be confused with P. enteroxantha, with which it also shares a paraplectenchymatous upper cortex. It can be distinguished from that species, however, by the distinctive shape of the soralia and the K- medulla. The soralia of Physconia enteroxantha are usually linear and continuous, and although they may be slightly or at times more strongly reflexed, they are neither hooded nor formed by separating cortices. Another similar species is P. leucoleiptes, rare in western North America, which has the same spot tests as P. fallax (soralia K+ and KC+ , medulla K-, KC ) but has distinctly labriform soralia which are not at all hooded by the cortices, and has a scleroplectenchymatous upper cortex. Physconia perisidiosa, which has terminal labriform or sometimes weakly hooded soralia, has a very different lower surface which is basically ecorticate and pale, darkening only centrally and never with a true cortex.