Thallus: foliose but may appear fruticose, composed of interwoven, dichotomously lobate lobes: irregular, narrow, sometimes several cm long, (0.5-) 1-1.5 (-4) mm wide, parallel-sided, not widening towards apices, with long mostly dichotomously branched sometimes squarrose cilia along the margins upper surface: whitish to cream-colored, shiny, without pruina;.sorediate soredia: in irregular soralia formed on the underside causing the lobes to widen upper cortex: prosoplectenchymatous, around half of the lobe thickness medulla: white, very thin lower cortex: absent lower surface: white throughout or partially pinkish brown (paricularly in the herbarium) or rarely purple, margin corticate and prominent (e.g. becoming canaliculate), erhizinate Apothecia: rather rare, subterminal, 2-3 (-5) mm diam., stipitate; disc: dark brown to black, ± pruinose, margin crenulate to lobulate, lobes up to 10 mm asci: cylindrical to subclavate, 8-spored ascospores: brown, l-septate, ellipsoid, Pachysporaria-type, (31-) 34.5-42.5 (-49.5) x (15-) 16-20 (-23.5) µm Pycnidia: rare, immersed conidia: bacilliform, 4-5 x 1 µm Spot tests: cortex K+ yellow C-, KC-, P+ yellow; medulla K- or K+ yellow to red, C-, KC-, P- or P+ orange Secondary metabolites: cortex with atranorin and chloroatranorin, medulla with atranorin, zeorin and ± salazinic acid. Substrate and ecology: growing both on trees and over mosses on rocks in both moist and fairly dry situations World distribution: one of the most widespread and abundant species throughout tropical and subtropical regions, extending to warm temperate areas Sonoran distribution: known from coastal localities in southern California and Baja California, the mountains in the southern tip of Baja California Sur and the Sierra Madre Occidental region of Chihuahua, Sonora and Sinaloa; the subsp. boryi occurs in Arizona, southern California, Chihuahua, Sonora and Baja California Sur. Notes: Heterodermia leucomela is easily characterized by the entangled mats of elongate, linear lobes bearing long black cilia and the ecorticate lower surface with ± thick corticate margins. The epithet lecuomelos (leucomelas), in the feminine should be leucomelaena, or it could be abbreviated as leucomela. Although Poelt in his combination correctly used leucomelaena, we prefer to use the abbreviated form leucomela as several publications since Poelts combination have used this form. It differs from H. linearis by the presence of soralia and when apothecia rarely are present they have long lobes on the margins. Heterodermia leucomela ssp. boryi (H. boryi) is included in the treatment.