Thallus: areolate, thick (0.3-0.8 mm), with rather wide fissures, without a black basal layer, with a distinct margin, without an apparent prothallus areoles: ±round-angular, plane to slightly convex, strongly wrinkled verrucose, (0.5-)1-2 mm wide surface: medium to dark brown, dull anatomy: upper cortex: poorly defined but with a brown uppermost cell layer sometimes overlain by a thin and irregular epinecral layer; algal layer: 150-200 µm deep, with densely packed algal cells 5-8 µm in diam., mycobiont subparaplectenchymatous, composed of cells 4-8 µm in diam.; medulla: with a looser texture, up to 600 µm thick, inspersed with rock fragments and crystals, colorless or patchily brown (especially around perithecia) Perithecia: 1-3 per areole, originating within the medulla, remaining immersed for a long time during their development, then in raised portions of the areoles, and finally with tips bursting through the surface; exciple: globose, 0.25-0.35 mm wide, colorless to brown; involucrellum: completely enveloping the perithecia, but basal parts lighter pigmented and more diffusely delimited, 50-70 µm thick; periphyses 40-50 µm long, thin, simple or sparingly furcate asci: saccate, 70-85 x 20-30 µm, 8-spored ascospores: hyaline, simple, ellipsoid, 23-27(-30) x 10-13(-15) µm Pycnidia: unknown Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: epilithic, on calcareous rocks World distribution: Europe (desribed from Finland) Sonoran distribution: one record from cenral Arizona (Maricopa County) and one from California (Orange Co.). Notes: The most characteristic feature of Verrucaria onegensis ist the development of the perithecia that originate in the medulla and are covered by thallus tissue for a long time. In thallus sections the perithecia are seen in different horizons within the thallus. Superficially similar species have their perithecia originating within the algal layer and, moreover, differ in having dimidiate involucrella and in several finer aspects: Verrucaria macrostoma has spores of similar size but a smooth, non-wrinkled, thallus surface, broadly pyriform perithecia and thicker, branched-anastomosing periphyses. Verrucaria murorum has larger spores. In Verrucaria maculicarpa the involucrellum may be in-curved beneath the exciple to partly closed but it is markedly thinner, and its spores are broader.