sp. nov. Diagnosis: Thallus foliosus, sorediatus; subtus albus vel pallido-brunnescens; conidia ±ve cylindrica, 712 x <1 µm. Thallus: foliose, up to 3 or 4 cm in diam., more or less orbicular lobes: elongate and more or less discrete to somewhat irregularly rounded and partly imbricate, 0.6-1.5 mm broad, usually +flat, prostrate upper surface: gray to mostly gray-brown, epruinose but with a more or less continuous and distinctive epinecral layer, which is distinctly patchy on the lobe ends (and therefore very pruina-like), also with sparse, tiny (up to 10-12 µm long & 1 cell thick) cortical hairs on the lobe tips, sorediate soredia: granular to pseudocorticate and isidioid, in primarily marginal or terminal on short side lobes (rarely laminal) soralia that become partly reflexed appearing weakly labriform or eventually almost capitate upper cortex: paraplectenchymatous medulla: white lower cortex: paraplectenchymatous lower surface: almost white to pale tan or occasionally pale brown in some older parts, dull; rhizines: simple or furcate, concolorous with the lower surface or sometimes darkening, usually conspicuous and numerous Apothecia: occasional, up to 1.5 mm in diam., sessile to short stipitate; margin: entire or becoming crenate in older ones, without a corona of rhizines ascospores: ellipsoid, (16-)18-25(-27) x 8-12 µm, Physcia-type Conidia: (7-)812 x <1 µm, cylindrical, although frequently slightly bent near the middle Spot tests: all negative in cortex and medulla Secondary metabolites: none detected. Substrate and ecology: usually bark or cactus stems, less often on rock World distribution: North America Sonoran distribution: states of Sonora and Puebla, Mexico. Notes: Among the larger sorediate species in this genus, the pale lower surface of P. culbersonii is shared only with P. nashii, which is also very similar in general appearance. When pycnidia are present (and they were found in every collection seen for this study), the distinctly different, long and cylindrical conidia of P. culbersonii will easily distinguish it from P. nashii (and all other Phaeophysciae). However, scrappy material or material lacking pycnidia, can be difficult to determine. The patchy, pruina-like, epinecral layer is more conspicuous in P. culbersonii, and the tiny cortical hairs present on lobe tips of that species (but easily overlooked and visible only on a dissecting scope under 30-40x or greater) are absent in P. nashii (although the roughness of the patchy epinecral layer on the lobe tips might be mistaken for tiny hairs). The conidia of this species are very different from every other species of Phaeophyscia, and it may deserve segregation as a separate genus. However, in most other characters, it is virtually identical to P. nashii, and so its disposition requires further study. This species is named for the collectors of the type specimen, my friends and mentors, William Louis Culberson and Chicita Culberson.