Thallus: foliose, irregular to orbicular, up to 3 cm diam., discretely lobate lobes: narrow, up to 1 mm, usually flat, not distinctly widening at tips, eciliate upper surface: white to cream-colored, shiny and without pruina, sorediate soredia: in labriform soralia, sometimes becoming very big (up to 5 mm tall) and spathulate upper cortex: thick, prosoplectenchymatous medulla: white lower cortex: absent lower surface: white on outer parts of lobes, pale brown in inner parts (appearing corticate, but in sections this proves to be medulla incrusted by soil particles); rhizines: sparse, marginal, c. 1 (-2) mm long, simple, sometimes projecting beyond the margin Apothecia: rare asci: subclavate to clavate, 8-spored ascospores: brown, l-septate, narrowly ellipsoid to ellipsoid, Pachysporaria-type, 36-43 x 15-18 µm Pycnidia: not seen Spot tests: cortex K+ yellow C-, KC-, P+ yellow; medulla K+ yellow, C-, KC-, P- Secondary metabolites: cortex with atranorin and chloroatranorin, medulla with atranorin, zeorin and an unidentified substance, UV+ red after charring rf 4-5 (G) ['spathulin']. Substrate and ecology: growing on both rocks and tree trunks in open but humid conditions World distribution: earlier known only from the Azores (Moberg and Purvis 1997) Sonoran distribution: not present in the Sonoran Desert area, but just outside the region and thus suspected to be found. Notes: This species is distinguished by its narrow, shiny, firmly adnate lobes with pale marginal rhizinae and unique chemistry, containing an unknown substance, spathulin, appearing reddish in UV above zeorin after treatment with sulfuric acid. If soralia are present they are usually big and spathulate. Another Heterodermia with small adnate lobes is H. albicans, which differs in having, darker lobes with small marginal knob-like projections developing into soralia and a very distinct lower cortex. In moist and shaded habitats the soredia of H. spathulifera tend to develop into squamules, that sometimes may cover inner parts of the thallus.