Thallus: squamulose, peltate, squamules 0.3-1.5 mm wide, sometimes up to 3-4 mm wide, squamules contiguous to imbricate, margins ascending to down-curved, sometimes wavy and lobulate, very brittle upper surface: dark olive brown, rarely somewhat grayish pruinose, dull or slightly glossy, at first smooth, later with some superficial cracks lower surface: pale, attached to the substrate by aglutinated tufts of rhizohyphae forming a tiny umbilicus thallus anatomy: homoiomerous, mycobiont cells short-celled to elongated, angulate to ±round, 5-10 x 3-5 µm; photobiont cells: 10-15 µm wide including gelatinous sheath; sheath: yellowish brown, hyaline inside, K-, 2-2.5 µm thick Apothecia: immersed to eventually semi-sessile, ±round, 0.25-0.4 mm wide disc: at first punctiform, soon open, dark red or yellowish brown to almost black, depressed, smooth thalline margin: thick, c. 50 µm wide, persisting, smooth; proper exciple: hyaline, thin, c. 15 µm epihymenium: yellowish brown, K- hymenium: hyaline, IKI+ blue, c. 100 µm tall; paraphyses: thin, distinctly septate, sparsely branched and anastomosing, straight, apical cells up 3-3.5 µm wide; subhymenium: hyaline, IKI+ blue asci: subclavate, thin walled, IKI-, (8-)12-16(-24)-spored ascospores: simple, hyaline, ellipsoid to broad fusiform with rounded poles, 9-12.5 x 4-6 µm, thin walled Pycnidia: not observed Spot tests: all negative Secondary products: none detected. Substrate and ecology: on desert soil and soft and crumbling or soil-incrusted rock in rocky slopes and open plain World and Sonoran distribution: Arizona, southern California, Baja California, Baja California Sur, and Sonora. Notes: Gloeoheppia squamulosa is identified among other soil-dwelling cyanobacterial lichens by its dark olive-brown thallus color, its homoiomerous, densely reticulate, its subgelatinous thallus anatomy with single-celled cyanobacterial photobionts, and its thin-walled, nonamyloid asci usually containing 12-16, ellipsoid to broadly fusiform spores with rounded poles. Gloeoheppia polyspora differs in its crustose growth form, in being restricted to rocks and in having 16-32, globose to subglose ascospores. Peltula differs in the stratified thallus anatomy, the asci having an amyloid apical dome and usually containing >32 spores. Heppia differs in the stratified thallus anatomy, type of photobiont and the strictly 8-spored asci. Pseudopeltula heppioides differs in its usually rough or dotted apothecial discs and the hymenium which is usually divided by sterile bands of hyphae, in its 8-spored asci and rocky substrate.