Life habit: sometimes appearing lichenicolous but often independent Thallus: starting as a fissured crust but developing contiguous to dispersed areolae or subsquamules, without a prothallus areoles/squamules: at first angular, then rounded, plane to slightly convex or wavy, 1-2 mm wide, up to 0.6 mm thick, usually widening from a narrower base, with margins becoming lobed and subascending to partly overlapping, with a base often elongate to form a stipe-like attaching organ surface: medium to usually dark brown, smooth, dull to somewhat shiny anatomy: upper cortex: paraplectenchymatous, composed of cells 5-8 µm in diam., 10-25 µm thick, with a brown uppermost cell layer overlain with an amorphous hyaline layer up to 20 µm thick; algal layer: diffusely delimited against the medulla, 100-200 µm thick, with algal cells 8-15 µm in diam.; alga-free medulla: subparaplectenchymatous , composed of roundish cells 7-10 µm in diam., below partly with filamentous hyphae froming hyaline hyphae c. 4 µm in diam. penetrating into the substrate Perithecia: immersed; exciple: subglobose, 0.25-0.40 mm wide, hyaline with only the apex brown-black, darkening throughout with age; involucrellum: lacking; periphyses 25-35 µm long, 2-3 µm thick, simple or furcate asci: (narrowly) clavate, 55-70 x 13-20 µm, 8-spored ascospores: hyaline, simple, broadly ellipsoid, 13-16 x 8-10 µm Pycnidia: immersed, 70-200 µm across conidia: bacilliform, 5-7 x c.1.5 µm Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: often epilithic, on both limestone and acidic rocks, often scattered among crustose lichens or partly parasitic thereon World distribution: widely distributed but scattered in Europe, Asia, North Africa, North America, Australia, and New Zealand Sonoran distribution: widely scattered in Arizona, southern California, and Baja California Sur. Notes: Verrucaria compacta as presently understood is a rather polymorphic species on various substrata that is possibly heterogeneous.