Thallus: cracked-areolate to subsquamulose, 0.15-0.20(-0.25) mm thick areoles: angular to irregularly roundish, plane to slightly convex, 0.3-0.6 mm wide surface: brown, dull to somewhat shiny anatomy: upper cortex c. 20 µm thick, composed of polygonal cells 5-7 µm in diam., overlain by amorphous, hyaline epinecralic layer 5-10 µm thick; algal layer: concentrated in upper part of the thallus or filling most of thallus thickness, with algal cells 6-12 µm in diam., mycobiont portion paraplectenchymatous; alga-free medulla: if well-defined at all, subparaplectenchymatous, colorless, inspersed with substrate particles Perithecia: 2-5 within each areole, fully immersed with only the tips visible from above as black dots; exciple: globose, 0.13-0.18 mm wide, colorless to brown-black, 10-15 µm thick, slightly thickened at the apex; involucrellum: lacking; periphyses 15-20 µm long, simple asci: clavate, 60-70 x 18-23, 8-spored ascospores: hyaline, simple, ellipsoid, 12-15 x 5-7 µm Pycnidia: unknown Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: epilithic, on calcareous rocks World distribution: southwestern North America, including New Mexico Sonoran distribution: Arizona and southern California. Notes: The type of Verrucaria rupicola was presumably destroyed with the herbarium of B. de Lesdain in Dunkirk, but the Sonoran material fits well with the original description: brown, areolate to subsquamulose thallus, dark-walled, small perithecia, and small ascospores. Verrucaria americana has broader spores and is silicolous. Placopyrenium insuetum is externally very similar, but differs in having thicker and larger thalli, areoles fastened by stipes, larger perithecia and longer ascospores (15-18 x 5-6 µm).