Thallus: crustose, thin, continuous, or more usually rimose-areolate, areoles up to 0.5-1.2(-1.6) mm wide, plane or rugose surface: light gray, dull; margin: determinate; prothallus: lacking; vegetative propagules: absent Apothecia: often erumpent, remaining adnate, frequent, often contiguous, up to 0.35-0.6(-0.7) mm in diam. disc: black, plane, becoming slightly convex thalline margin: concolorous with thallus, c. 0.1 mm wide, entire, subcrenulate in thicker thalli, persistent or often incompletely developed in part, then replaced by a thin, pigmented proper exciple; excipular ring: otherwise absent thalline exciple: 60-110 Ám wide; cortex: 5-10 Ám wide; cells: up to 3.5-6 Ám wide, not pigmented; algal cells: up to 11-15 Ám in diam. proper exciple: hyaline, 5-10 Ám wide laterally, expanding to 10-25 Ám wide at periphery hymenium: 60-110 Ám tall; paraphyses: 1.5-2 Ám wide, often conglutinate, with apices expanded up to 3.5-5 Ám wide, lightly pigmented, immersed in dispersed pigment, forming a red-brown epihymenium; hypothecium: (20-)60-80(-100) Ám thick asci: clavate, 50-60 x 14-18 Ám, 8-spored ascospores: ellipsoid, brown, 1-septate, developmental Type A, Physcia-type, (13-)16-17(-19.5) x (7-)8-9(-10) Ám, lumina persistently angular or finally becoming rounded, maintaining thick apical wall; torus: becoming darkly pigmented; walls: not ornamented Pycnidia: infrequent, deeply immersed in thallus with only pigmented ostiole margin visible; conidiophores: Type II; conidia: bacilliform, 3.5-5 x c. 1 Ám Spot tests: K- or mostly K+ yellow, C-, KC-, P- or P+ faint yellow Secondary substances: atranorin in cortex. Substrate and ecology: on conifers and deciduous trees, and rarely on wood World distribution: a western North American endemic that has a Rocky Mountain distribution extending from southern Alberta to Colorado and into Arizona where it has been most frequently collected Sonoran distribution: Arizona and Baja California, where it occurs at its highest known elevation of 2800 m. Notes: The light gray, K+ yellow thalli of R. boulderensis, often with slightly convex apothecia, have a similar habit to R. aurantiaca and R. capensis. Both these species are easily distinguished by possessing apothecia with a columnar lower cortex and larger spores. Rinodina boulderensis can usually be recognized by its relatively small apothecia and their often incompletely formed thalline margins. The spores of R. glauca are similar in size, also possess a darkly pigmented torus and lack wall ornamentation, and taken together these may indicate an evolutionary relationship. Rinodina glauca is easily distinguished by its dark gray thallus and lack of atranorin. Rinodina boulderensis may be related to R. exigua (Ach.) Gray, whose occurrence in North America is questionable. Rinodina exigua is distinguished by possessing spores, that have a less well developed and less pigmented torus, and apothecial margins, that are always fully developed but may become excluded.