Thallus: crustose, areolate to squamulose; prothallus: not visible areoles: flat to subsquamulose to squamulose (at least the marginal parts), often irregular in shape, up to 2 (-5 mm) in diam, with marginal areoles often slightly lobed (effigurate), sometimes irregularly folded, 0.5-0.7(-1.3) mm thick surface: pale beige brown to pale olive-gray, dull to faintly glossy, with a small white rim, esorediate medulla: white, I-, C- lower surface: black near edges Apothecia: black, young ones aspicilioid (immersed within the areoles), large ones sunken in between the areoles, without a constricted base, +emarginate (except very young ones), up to 0.9(-2.5) mm in diam. disc: black, flat to moderately convex, epruinose to conspicuously pruinose margin and exciple: usually almost completely reduced epihymenium: green to green-black (cinereorufa-green), more rarely dirty brown, 10-20 µm thick hymenium: hyaline to pale green, 50-75 µm tall; paraphyses: simple, 3-4 µm wide, rarely branched and anastomosing, with inconspicuous apical cells 3.5-6 µm in diam. subhymenium: unpigmented (discernible only after I treatment), 10-35 µm thick hypothecium: blackish brown to more rarely pale brown, 40-250 µm thick asci: clavate, 50-65 x 11-13 µm, with Lecanora-type tholi, 8-spored ascospores: hyaline, simple, ellipsoid, (6.5-)9.5-13.5 (-17) x (4-)4.8-6.4(-8) µm Pycnidia: immersed conidia: filiform, +straight, (15-)18-25(-30) x 1-1.2 µm Spot tests: medulla K+ yellow, C-, KC-, P+ deep yellow Secondary metabolites: atranorin and psoromic acid. Substrate and ecology: on calcium free boulders in open habitats, at 2502200 m, often associated with Candelina species World and Sonoran distribution: southern Arizona and widely distributed in Chihuahua, Sonora, Sinaloa (and Durango), Baja California, and Baja California Sur. Notes: Lecidea pseudaglaea belongs to the Lecanoraceae; its generic position within this family remains unclear. It is characterized by a thallus of the L. atrobrunnea/fuscoatra type (areolate to squamiform, smooth to shiny, due to an epinecrotic cortex-layer), lecideoid apothecia, a tall and (usually dark) brown hypothecium, and its chemistry. Most of the herbarium specimens were filed under the name "Lecidea subaglaea de Lesd." Unfortunately we could not study type material [Mexico, Nevada de Toluca, 4400 m, G. Arsene 823] of Lecidea subaglaea. According to the protologue Lecidea subaglaea is characterized by a white, dull thallus with a conspicuous black hypothallus, an unpigmented hypothecium, wider spores and it is, in strong contrast to Lecidea pseudaglaea, an alpine element.