Thallus: squamulose squamules: 1-2 mm wide, 0.2-0.35 mm thick, solitary to adjacent and hardly overlapping, adnate, plane or with weakly elevated margins, shallowly to deeply incised upper surface: grayish brown to medium brown, the edges often paler and appearomg as though gnawed upper cortex: 30-50 Ám thick, composed of anticlinally oriented hyphae; cells: in vertical columns, angular, isodiametrical and 5-10 Ám in diam., or vertically elongated and up to 13 x 7 Ám, amorphous layer up to 30 Ám high medulla: white, composed of interwoven hyphae divided into a varying number of spherical cells (7-11 Ám in diam.); algal layer: c. 60-100 Ám thick, unevenly delimited; algal cells: 7-12 Ám in diam. lower cortex: no true one developed, but more densely packed spherical cells found below lower surface: whitish or darkening, attachment by rhizohyphal wefts and additionally rhizines; rhizohyphae: hyaline, 3-4.5 Ám in diam., forming a rather loose hypothalline weft; rhizines: few per squamule, whitish or pale gray-brown, normally not blackening, long and slender, sparingly branched; main stems: 50-120 Ám thick and up to 3 mm long, not linking squamules Perithecia: broadly pyriform to almost globose, up to 0.45 mm broad; exciple: black, 20-30 Ám thick; periphyses: 25-40 Ám long; hymenial algal cells: subglobose to ellipsoid, 3-5 Ám in diam. asci: oblong-clavate, 75-95 x 20-28 Ám, 2-spored ascospores: muriform, pale to dark brown, 30-41 x 17-25/ 38-54 x 13-21 Ám (distal/proximal spores) Pycnidia: rarely found conidia: c. 5 x 1 Ám Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: in arid sites on fine-grained soil, rarely on weathered rock World distribution: Europe (Spain, Austria, Russia) and western North America (from Washington south to Mexico) Sonoran distribution: Arizona, southern California and Baja California Sur. Notes: Endocarpon loscosii resembles E. pusillum from which it is distinguished by pale undersides of squamules and whitish rhizines. Though the rhizines may darken with age, they never become carbonaceous as in E. pusillum. Moreover, the smaller asci are distinctive (75-95 x 20-28 Ám versus 90-125 x 20-33 Ám in E. pusillum).