Thallus: deeply areolate, 0.3-0.6 mm thick, up to 6 cm across, usually with a thick carbonaceous basal layer, with a thinner margin bordered by a black prothalline line areoles: angular, 0.5-1 mm wide, when fertile, sterile ones 0.1-0.3 mm, with rather broad cracks, dark-rimmed or not, with black side-walls, secondarily subdivided into smaller units by thin dark lines surface: gray to pale brown, weakly to strongly whitish gray pruinose, dull, ±crossed by dark lines along which the areoles tend to subdivide anatomy: upper cortex: poorly defined but consisteing of 1-3 layers of small cells with the uppermost one brown pigmented, sometimes overlain by a thin, amorphous epinecral layer; algal layer: disrupted by black tissue -hence forming small pockets of algae up to 150 µm thick, with algal cells 7-12(-15) µm in diam., mycobiont portion subparaplectenchymatous, with a thick friable, carbonaceous basal layer usually accounting for half or more of the thallus thickness and without mineral inclusions Perithecia: 1-2 per areole, arising within the algal units in larger and often less subdivided areoles, fully immersed, their bases sunken in the black basal layer, appearing at thallus surface as black spots which are plane or slightly convex, up to 0.3(-0.4) mm across, round or somewhat angular in outline; exciple: subglobose, 0.2-0.4 mm wide, dark brown to black, scarcely distinguishable from the involucrellum; appressed to the exciple, lateral 40-70(-100) µm thick, extending to the exciple-base level and merging with the black basal layer; periphyses 30-40 µm long, thin, simple to sparingly branched asci: clavate, 70-80 x 25-30 µm, 8-spored ascospores: hyaline, simple, oblong-ellipsoid, (18-)20-27(-30) x (7-)8-11(-12) µm, with a thin perispore Pycnidia: unknown Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: epilithic, on different kinds of acidic or calcareous rocks (basalt, andesite, sandstone, caliche) in a broad range of habitats from coastal sites to moderate altitudes World distribution: southwestern North America Sonoran distribution: widely distributed and rather common in Baja California and southern California, mainly coastal, many collections from the Channel Islands. Notes: The spores of Verrucaria subdivisa vary considerably in size. With the dark basal layer and the algal parts forming islands which are basally and laterally surrounded by black tissue, the species is similar to Verrucaria nigrescens, but the latter has brown, epruinose thalli. The carbonaceous basal layer is usually thick, but may be inconspicuous in thin thalli. For differences to other species of the Verrucaria fuscella group see under V. alutacea. Verrucaria subglaucina de Lesd., described from New Mexico, differs in lacking a carbonaceous basal layer, in having an apical involucrellum, and broader spores.