Thallus: appressed to suberect or trailing, up to 4 (-6) cm broad or long; texture: cartilaginous; branching: variable; budding and adventitious lobes: present lobes: separate to imbricate, 0.5-2 (-3) mm broad, sometimes black bordered; profile: even to ± nodulose; width/height ratio: 1-2; tips and axils: sparsely perforate upper surface: white to greenish gray, sometimes dark mottled, smooth to weakly rugose; soredia and isidia: absent; lobules: often present medulla: hollow, ceiling of cavity dark, floor of cavity dark lower surface: black, sparsely perforate Apothecia: common, substipitate to stipitate, up to 7 (-9) mm in diam; stipe: urn- or funnel-shaped, hollow; disc: reddish brown to dark brown ascospores: ellipsoid to broadly ellipsoid, 5-7.5 x 3-3.5 µm Pycnidia: common conidia: rod-shaped to weakly bifusiform, 5.5-8 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), 3-hydroxyphysodic acid, and unknown C7 (UV+, minor). Substrate and ecology: on bark of conifers World and Sonoran distribution: endemic to Guadalupe Island, Mexico. Notes: Closely related to H. heterophylla, H. guadalupensis is distinguished by its P- medulla (physodalic acid absent) and the presence of 3-hydroxyphysodic acid. The habit is also somewhat different with H. heterophylla usually having semi-erect to erect lobes, while the specimens of H. guadalupensis known so far have a more drooping, lax habit. H. guadalupensis is also similar to H. inactiva, but H. inactiva usually lacks the adventitious branching, 3-hydroxyphysodic acid, and unknown C7 of H. guadalupensis. The only known collections of H. guadalupensis were made by Howell in 1931 on Guadalupe Island. The exact location is unknown. More recent collections of Hypogymnia from the island are mainly H. imshaugii, with a bit of H. schizidiata. Island endemics are also known in the genus from the Canary Islands and Madeira [H. tavaresii D. Hawksw. & P. James and H. madeirensis (Tav.) D. Hawksw.].
Because there are so few specimens, perhaps collected at a single site, could this be considered an aberrant population of H. heterophylla or another species. A large population can be inferred from the number of individuals that Howell collected. Few others have collected lichens on Guadalupe Island. Despite the shortage of data and small known range, H. guadalupensis is distinct from all the other material on the offshore islands, it is easy to identify, and it does not appear to intergrade with other species. Howells collections could not be accommodated within existing species without significantly expanding our concept of H. inactiva or H. heterophylla.