Thallus: squamulose squamules: 1-1.5 mm wide, 0.2-0.25 mm thick, plane, contiguous to ± overlapping, incised upper surface: brown, smooth, dull upper cortex: c. 30 µm thick, cells 5-8 µm wide medulla: white, subparaplectenchymatous, composed of roundish cells (5-8 µm in diam.), with small interhyphal spaces; algal layer: c. 60-80 µm thick; algal cells: 5-11 µm in diam. lower cortex: thin, composed of 1-3 layers of roundish-angular cells (5-7 µm in diam. with brown-black walls); rhizohyphae: hyaline to faintly brown, 4-5 µm thick lower surface: pale brown Perithecia: broadly pyriform, up to 0.3 mm broad; exciple: at first pale but soon darkening, black in old perithecia; periphyses: stout, 15-25 µm long; hymenial algal cells: subglobose, 3-5 µm in diam. asci: clavate, c. 65-85 x 17-22 µm, 2-spored ascospores: muriform, colorless to pale brownish, 25-38 x 15-18/ 30-42 x 13-16 µm (distalproximal spores) Pycnidia: rarely found conidia: bacilliform, c. 5 x 1 µm Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: on rock (schist or limestone) World distribution: previously known only from Italy (Liguria) Sonoran distribution: collected once in Arizona. Notes: Endocarpon schisticola is a rare and little-known species characterized by small saxicolous squamules with pale undersides, and short periphyses. From the other small, saxicolous species of the region (E. pallidulum and E. petrolepideum) it is separable, besides its stout periphyses, by its thicker rhizohyphae and larger hymenial algal cells.