Thallus: crustose, rimose to rimose-areolate, thin, ±continuous; prothallus: distinct, delimiting the thallus as a black outline surface: usually pale greenish yellow, rarely gray (older herbarium specimens), dull, smooth, epruinose, phenocorticate, esorediate medulla: white, lacking calcium oxalate (H2SO4-) Apothecia: lecideine; (0.3-)0.4-0.9(-1.8) mm in diam., soon sessile margin: black, conspicuously thickened and frequently fractured, always persistent, not excluded with age disc: black, epruinose, plane, not becoming convex with age proper exciple: very thick, distinctly differentiated into an inner and outer part, trachyspora-type , i.e., inner excipular hyphae large, leptodermatous, ±isodiametric cells, reddish brown, paraplectenchymatous (textura angularis), transient with the deep reddish brown hypothecium (leptoclinoidesbrown, textura intricata), outer excipular hyphae short-celled, cells globose, (textura globularis) and extremely carbonized with large amounts of a brown pigment (cf. elachista-brown HNO3-) epihymenium: brown, pigmentation continuous with the outer exciple (HNO3-) hymenium: hyaline, not inspersed with oil droplets; paraphyses: simple to moderately branched, apically swollen, with a brown pigment cap (cf. elachista-brown) asci: clavate, Bacidia-type, 8-spored ascospores: soon brown, 1-septate (occasionally additional septa forming at both ends of the spore cells), oblong to ellipsoid, sometimes constricted, with obtuse ends, not curved, (14-)18.8-[22.6]-26.5(-34) x (8-)8.7-[9.6] 10.5(-12) µm (n=60); proper septum: narrow, not thickening during spore ontogeny (Buellia-type); ornamentation: coarsely rugulate to areolate (visible at 200x magnification, without DIC) Pycnidia: rare to common, urceolate to globose, unilocular; ontogeny similar to the Umbilicaria-type conidiogenous cells: mostly terminal, rarely also intercalary (cf. conidiophore-type V) conidia: short to elongate bacilliform, 10-13 x 0.5-1 µm (n=20) Spot tests: all negative (K-, P-, C-, KC-, CK-) fluorescence: UV+ bright orange iodine reaction: medulla non-amyloid Secondary metabolites: 4,5-dichloro-3-O-methylnorlichexanthone, 4,5-dichlorolichexanthone (J. A. Elix, HPLC). Substrate and ecology: epilithic, on sheltered siliceous rock (HCl-), usually close to rivers and streams World distribution: subtropical-tropical of the Caribbean, including the Virgin Islands, Cuba, Jamaica, Puerto Rico, and Mexico Sonoran distribution: a single collection from southwestern Chihuahua, another one from south of the region (Nayarit, Mexico). Notes: Although B. trachyspora has a unique apothecial anatomy, it has been confused with B. mamillana. Thalli of the two species are indeed very similar, but young apothecia of B. trachyspora emerge early during its ontogeny and are never remotely lecanorine (not even a thalline collar or a thalline veil has been observed in the material).