Thallus: suberect, up to 3 (-5) cm broad; lobe tips: often upturned; texture: cartilaginous; branching: variable; budding and adventitious lobes: usually present lobes: separate, occasionally centrally contiguous, (0.5-) 1.0-1.5 (2.5) mm wide; black border: sometimes visible from above; profile: ± even; width/height ratio: (0.6-) 1-2 (-4); tips and axils: perforate below (perforations always present but not all lobe tips and axils are perforate) upper surface: white to greenish gray, usually dark mottled, smooth to rugose; soredia and isidia: lacking; lobules: sparse or lacking medulla: hollow but sometimes partially collapsed, ceiling of cavity white or dusky, floor white or dirty white lower surface: black, sparsely perforate Apothecia: common, substipitate to stipitate, up to 4 (-7) mm in diam; stipe: urn- or funnel-shaped, hollow, disc: brown to dark brown ascospores: subglobose, 7.0-7.5 x 4.5-5.0 µm Pycnidia: common conidia: weakly bifusiform, 5.5-8.5 x 0.6-0.7 µm Spot tests: cortex K+ yellow, C-, KC-, P+ pale yellow, UV-; medulla K-, C-, KC+ orange-red, P+ orange-red Secondary metabolites: upper cortex with atranorin and chloroatranorin; medulla with physodic acid (major), physodalic acid (major), protocetraric acid (minor), and unknown C7 (UV+, minor but constant), rarely with 3-hydroxyphysodic acid. Substrate and ecology: on bark of shrubs, conifers, and hardwoods, mainly near the coast in chaparral and open woodlands World and Sonoran distribution: endemic to southern California, adjoining Mexico, and offshore islands, common in coastal chaparral in Santa Barbara and San Luis Obispo Counties. Mainland populations are mostly within 10 km of the coast, with scattered records farther inland. Notes: This species is easily recognized by its whitish cavity, slender suberect separate lobes, and small holes below the tips and axils. Most collections were previously identified as H. imshaugii. However, typical H. imshaugii is imperforate and has a snow white medullary cavity. In contrast, H. gracilis typically has 20% or more of the lobe axils and tips perforate below and the lobe cavity varies from white to light brownish or light grayish. The cavity of H. imshaugii darkens only when the thallus is damaged; likewise, the rare perforations seen in the lower surface and lobe tips of H. imshaugii appear to result from herbivory. However, one of the forms of H. imshaugii, almost certain to be separated as a distinct taxon in the future, has occasional perforations, but it has collapsed or semisolid, rugose, non-mottled lobes. Despite the resemblance of H. gracilis to H. imshaugii, it is more closely related to H. heterophylla. They share the frequent presence of adventitious lobes and the presence of unknown C7 (actually a pair of UV+ spots with TLC, constant in both H. heterophylla and H. gracilis). Hypogymnia heterophylla, however, always has a dark brown to blackish medullary cavity. Furthermore, it is often much larger than H. gracilis and has slightly narrower spores. Hypogymnia gracilis can be distinguished from H. guadalupensis by the formers smaller size, P+ medulla, broader spores, and more erect thallus.