Thallus: squamulose squamules: 0.5-1.5 mm wide, 0.1-0.2 mm thick, loosely aggregated to imbricate, ± finely lobed, with margins free from the substrate and ± ascending, centrally attached upper surface: pale brown, smooth upper cortex: 20-40 µm thick, composed of rather large, angular cells (6-15 µm in diam.), lacking an amorphous layer medulla: white, subparaplectenchymatous, composed of spherical cells (5-8 µm in diam.), becoming more closely packed with depth to form a paraplectenchymatous basal layer; algal layer: c. 40-60 µm thick, rather continuous; algal cells: 6-11 µm in diam. lower cortex: weakly differentiated from the medulla, composed of more angular, closely packed and dark pigmented cells; rhizohyphae: hyaline to brownish, 2.5-3.5 µm thick, forming a few bundles (but not true rhizines) lower surface: black except for elevated marginal parts Perithecia: subglobose, up to 0.3 mm wide, causing swellings on the lower side of the squamules; exciple: black, c. 20 µm thick; periphyses: 20-35 µm long; hymenial algal cells: globose, 2.5-3.5 µm in diam. asci: clavate, c. 60-75 x 15-20 µm, 2-spored ascospores: muriform, hyaline to faintly yellowish-brown, 25-30 x 12-15/ 27-35 x 11 - 13 µm (distal/proximal spores) Pycnidia: not seen Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: on limestone and acidic rock or thin soil layer thereon World distribution: Peru, West Indies, SW North America, Queensland and Japan Sonoran distribution: Arizona, southern California, Sonora and Baja California Sur. Notes: The distinction from Endocarpon petrolepideum may be difficult. The squamules of E. pallidulum tend to be somewhat larger and more or less imbricate, and are attached to the substrate only by their central parts, whereas those of E. petrolepideum are smaller, ± dispersed and broadly adnate.