Thallus: foliose, 2-5 cm in diam., adnate to loosely adnate, subdichotomously lobate lobes: sublinear, elongate, usually convex, initially separate, becoming weakly imbricate, 0.5-2 mm wide, apices subtruncate, revolute to plane upper surface: light to greenish gray, smooth, dull to shiny, emaculate isidia: laminal, cylindrical; tips darker than thallus surface; soralia, pustulae and dactyls absent medulla: white with continuous algal layer lower surface: black with peripherally brown tips; rhizines: abundant but not at margins, black, dichotomous Apothecia: rare, laminal, substipitate, 3-8 mm in diam.; disc: brown; margin: smooth to crenulate, rarely isidiate asci: clavate, 8-spored ascospores: broadly ellipsoid, 14-18 x 8-10 µm Pycnidia: laminal, immersed conidia: not seen Spot tests: upper cortex K+ yellow, C-, KC-, P+ yellow, UV-; medulla K-, C-, KC+ rose to red, P-, UV+ white Secondary metabolites: upper cortex with atranorin and chloroatranorin; medulla with alectoronic acid (major), α-collatolic acid (minor), ß-alectoronic and α-collatolic acids (trace). Substrate and ecology: on various acidic rocks, frequently with a layer of detritus between the specimen and the rock in lower montane to montane regions, particularly in sheltered canyons World distribution: neotropics and extending into SW USA Sonoran distribution: in shaded canyons throughout SE Arizona southwards through the Sierra Madre Occidental of Sonora, Chihuahua and Sinaloa and in southern mountains of Baja California Sur. Notes: Initially these specimens were determined as H. ensifolia (Kurok.) Hale and confirmed by Hale. The presence of isidia and similar chemistry was consistent with this evaluation. However, the availability of a wider range of specimens reveals substantial morphological differences between H. ensifolia and H. meridensis. H. ensifolia is a substantially larger speices with more linear, wider lobes, marginal lobulae (or lacinae) and a looser attachment to its substrate. Secondary lobes in H. ensifolia are not as well developed as in H. meridensis and do not exhibit the typical dichotomous branching of H. meridensis.