Thallus: foliose, loosely adnate, pulvinate, 2-4 cm in diam., dichotomously lobate lobes: sublinear, elongate, plane to subconvex, separate and imbricate, 0.3-0.8 mm wide, developing marginal lobulae; apices: subtruncate, +ascending, smooth to crenate, black rimmed, eciliate upper surface: dark yellow-green, sometimes darkening, smooth but developing transverse cracks with age, shiny or dull, epruinose and emaculate, without soralia, isidia, or pustulae medulla: white, with continuous algal layer lower surface: black, plane to finely rugulose, very sparsely rhizinate; rhizines: black, simple, 0.2-0.3 mm long Apothecia: rare, substipitate, 2-4 mm wide, laminal on thallus; disc: cinnamon-brown to dark brown; margin: smooth, pruina absent asci: clavate, 8-spored ascospores: hyaline, simple, ellipsoid, poorly developed, 9 x 5 Ám Pycnidia: not observed Spot tests: upper cortex K+ yellow to orange, C-, KC-, P+ orange; medulla K+ yellow becoming dark red, C-, KC-, P+ orange Secondary metabolites: upper cortex with usnic acid (major); medulla with stictic acid (major) and constictic, cryptostictic and norstictic acids (all minor) and connorstictic acid (trace). Substrate and ecology: on acidic rocks or soil, often in semi-open, forested habitats World distribution: southwestern North America, including Mexico Sonoran distribution: occasionally at higher elevations in Arizona in mixed conifer forests. Notes: Although initially only known from a few localities in New Mexico (Hale 1990), we have now found X. planilobata several times in Arizona. Although Xanthoparmeliae are common at the upper montane elevations where this species occurs, the occurrence of species with a black lower surface is unusual in these populations. This very narrow lobed species does not appear to intergrade with any of the broader lobed species (e.g. X. neoconspersa).