Thallus: crustose, thin, epiphytic on lichen species on rocks, at first consisting of contiguous granules on host, 0.1-0.2 mm in diam., finally areolate, areoles up to 0.6-0.7 mm wide, plane or rugose surface: gray-brown (if shaded), more typically dark brown, dull; margin: mostly indeterminate, finally determinate; prothallus: present when finally autonomous, dark, fimbriate; vegetative propagules: absent Apothecia: innate to sessile, frequent and often contiguous, up to 0.3-0.7 mm in diam. disc: dark brown to black, plane or slightly convex thalline margin: concolorous with the thallus, sometimes incompletely formed, then appearing crenulate, 0.05-0.1 mm wide, otherwise entire and persistent; excipular ring: sometimes present, raised thalline exciple: 40-90 µm wide laterally; cortex: 5-15 µm wide; cells: up to 4-7.5 µm wide, pigmented red-brown; algal cells: up to 7.5-14.5 µm in diam.; thalline exciple: 40-110 µm thick below; cortex: 10-20(-40), hyphae intricate when >20 µm thick proper exciple: hyaline, 5-15 µm wide, expanding to (10-)20-40 µm at periphery hymenium: 70-90 µm tall; paraphyses: 2-3 µm wide, sometimes conglutinate, with apices up to 4-6 µm wide, heavily pigmented or not, immersed in diffuse pigment, forming a light, or more typically dark, red-brown epihymenium; hypothecium: 50-90 µm thick asci: clavate, 50-65 x 15-17 µm, 8-spored ascospores: brown, 1-septate, broadly ellipsoid, type A development, Physcia-type, (13.5-)16-16.5(-19) x (7-)8.5-9(-10.5) µm, lumina mostly retaining thickened apical walls, sometimes becoming thin walled (Physconia-like); torus: present, sometimes lightly pigmented; walls: lightly ornamented in mature spores, more strongly ornamented in some overmature spores Pycnidia: immersed in thallus; conidiophores: type I conidia: bacilliform, 3.5-4.5 x c. 1 µm Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: epiphytic on crustose lichens over rocks, mostly on Aspicilia species, but also becoming independent on granite and basalt World distribution: Greenland, European Alps, Scandinavia, Mongolia and western North America (southern Rocky Mountains, coastal ranges and Sierra Nevada) Sonoran distribution: Arizona, mountains of Gila and Santa Cruz counties, at elevations of 1900-2320 m. Notes: The thallus of R. parasitica is epiphytic on other crustose lichens and extremely variable due to its developmental sequence. In its colonization stage, it is reminiscent of Amandinea cacuminum (Th. Fr.) H. Mayrh. & Sheard, the apothecia being borne erratically on a granular thallus. The host species is gradually replaced and an areolate thallus finally lives independently when it may be limited by a black, fimbriate prothallus.