Thallus: appressed to suberect, up to 4 (-8) cm in diam.; texture: cartilaginous; branching: isotomic dichotomous; budding: absent or rare lobes: separate to centrally subcontiguous, 1-3 (-4) mm broad; black border: not visible from above; profile: even to ± nodulose; width/height ratio: 1-4; tips and axils: perforate, upper surface: greenish gray, greenish, or dark brownish green, often dark mottled, smooth or becoming strongly rugose soredia: sometimes developing on the edges of the schizidia, particularly in rugose areas of the upper surface, rarely spreading into diffuse laminal soredia; isidia: absent; lobules: rare schizidia: formed as flakes of cortex plus algal layer, developing from either the smooth or rugose areas of the thallus medulla: hollow, ceiling of cavity brownish to white, floor of cavity brownish to white lower surface: black, sparsely perforate Apothecia: occasional, substipitate to stipitate, up to 6 mm diam; stipe: urn- to funnel-shaped, hollow; disc: light to dark brown ascospores: ellipsoid, (5.5-) 6.5-7 (-8) x (3.5-) 4-5 µm Pycnidia: common conidia: rod-shaped to weakly bifusiform, 6-7.5 x 0.5-0.7 µm Spot tests: cortex K+ yellow, C-, KC-, P+ pale yellow, UV-; medulla K-, C-, KC+ orange-red, P- Secondary metabolites: upper cortex with atranorin and chloroatranorin; medulla with physodic acid (major), 2'-O-methylphysodic acid (minor), 3-hydroxyphysodic acid (accessory, frequency 25%), unknown C7 (minor UV+ accessory), unknown C8 (minor accessory). Substrate and ecology: on bark and wood of both hardwoods and conifers, including Juniperus, Pachycormus, Quercus, Pinus, and Simmondsia in woodlands, isolated groves, and forests World and Sonoran distribution: offshore islands of Baja California (Isla Cedros) and California (Santa Rosa Island). Notes: Production of asexual propagules is rather variable, but in most specimens the cortex tends flake off from both smooth parts and rugose parts, taking with it the algal layer. The type is not as heavily schizidiate as some specimens, but it is fertile and shows the variation from smooth to rugose lobes. True soredia can be nonexistent to well developed. The species is also unusual in its light brownish lobe cavities. In most specimens it is somewhat variable, having pure white portions, occasional dark brown portions, but the majority of the lobe interiors have a loose mesh of brown hyphae over a white background. This plus the frequently perforate lobe tips separate the species from H. imshaugii. The P- medulla is also useful in separating H. schizidiata from the P+ chemotype of H. imshaugii. The somewhat darkened lobe cavities and perforations suggest H. inactiva, another P- species, but that species lacks asexual propagules and differs in accessory seconddary metabolites. Small presorediate specimens can be recognized by their brownish cavities, isotomic dichotomous branching, and perforations in the lobe tips and lower surface.