Thallus: foliose, loosely adnate, pulvinate, 1-3 cm in diam., subdichotomously lobate lobes: sublinear, ±elongate, plane to subconvex, separate and imbricate, 0.3-0.8 mm wide, developing marginal lobulae; apices: subtruncate, smooth to crenate, black rimmed, eciliate upper surface: 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: ivory to pale brown, darkening at apices, plane, very sparsely rhizinate; rhizines: concolorous or blackening, simple, 0.05-0.1 mm long Apothecia: only vestigial, immature apothecia seen Pycnidia: common, immersed conidia: bifusiform, 5-6 x 1 µm Spot tests: upper cortex K-, C-, KC-, P-; medulla K-, C-, KC-, P+ red Secondary metabolites: upper cortex with usnic acid (major); medulla with fumarprotocetraric acid (major), protocetraric acid (minor), confumarprotocetraric acid (trace), succinprotocetraric acid (trace). Substrate and ecology: on acidic rocks World distribution: southwestern North America Sonoran distribution: Arizona, Chihuahua and Sinaloa. Notes: In overall morphology this new species resembles X. planilobata but can readily be distinguished by the ivory to pale brown lower surface (black in X. planilobata) and the presence of medullary fumarprotocetraric acid (stictic acid present in X. planilobata). Xanthoparmelia tuckeriana could also be confused with X. monticola, a species with similar chemistry (but containing additional constipatic, protoconstipatic and physodalic acids), but the latter species has broader lobes (1-2.5 mm vs. 0.3-0.8 mm wide) with a moderately rhizinate lower surface and longer rhizines (0.5-1 mm vs. 0.05-0.2 mm). This species is named in honor of Dr Shirley Tucker in recognition of her many contributions to North American lichenology.