sp. nov. Diagnosis: Thallus foliosus, sorediatus; subtus albus vel pallido-brunnescens; conidia ellipsoidea, 2.5-4 x 1 µm. Thallus: foliose, up to 4(-5) cm in diam., +orbicular lobes: elongate and +discrete to truncate or more rounded-crenate, 0.5-1(-1.5) mm broad, usually +flat, prostrate upper surface: pale gray to brownish gray, epruinose but with an indistinct epinecral layer over much of the upper surface (being slightly patchy on the lobe ends), sorediate soredia: granular to pseudocorticate and isidioid, in primarily marginal or terminal on short side lobes (rarely laminal) soralia that become partly reflexed and appearing weakly labriform or eventually almost capitate upper cortex: paraplectenchymatous medulla: white lower cortex: paraplectenchymatous lower surface: white to pale tan, dull; rhizines: simple, concolorous with the lower surface, rather sparse Apothecia: frequent, up to 1.8 mm in diam., sessile to short stipitate; margin: entire or becoming lobulate; corona of rhizines: absent or inconspicuous ascospores: ellipsoid, 18-22.5 x 7-10.5 µm, Physcia-type Spot tests: all negative in cortex and medulla Secondary metabolites: none detected. Substrate and ecology: on bark World distribution: western Mexico Sonoran distribution: Sonora and Baja California Sur. Notes: Both P. insignis and P. nigricans might be confused with Phaeophyscia nashii because of their pale lower surface, although they are both smaller and distinguished by rather different soralia. Probably the most easily confused species is P. culbersonii, which is very similar in form, but can be distinguished from the present species by a more distinct patchy epinecral layer on the lobes (appearing more like a pruina) and the presence of very tiny cortical hairs (visible at high magnification dissecting scope) on the lobe tips. Also, the pycnidia that are almost always present in P. culbersonii, contain much larger conidia, 7-12 µm long. This species is named in honor of Thomas H. Nash III, foremost facilitator of the Sonoran lichen flora.