Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2007. Lichen Flora of the Greater Sonoran Desert Region. Vol 3.
Thallus: shrubby, usually short, 3 to 6 cm long, ±stiff branching: anisotomic-dichotomous, divergent basal part: concolorous with the branches to pale branches: main ones: irregular, distinctly segmented, ±swollen; lateral branches: not narrowed at point of attachment segments: terete to strongly ridged, cylindrical to ± sausage-like; transversal furrows/foveoles: numerous on main branches papillae: absent (can be mistaken with young fibrils) tubercles: absent fibercles: nearly absent to numerous fibrils: short and thick, spinulous, numerous and crowded (80-100 fibrils/0.5 cm on main branches) isidiomorphs: absent cortex: thin (3-7%), distinctly shiny, often distinctly cracked on main branches medulla: dense to compact, often partly chalky, with subcortical orange to red orange pigment, sometimes very faint, rarely absent axis: thin to wide Apothecia: often on short lateral branches, sometimes terminal to subterminal, 2 to 15 mm in diam. ascospores: ellipsoid, 9-11 x 5-7 µm Spot tests: medulla K-, C-, KC-, P- Secondary metabolites: usnic acid, ±fatty acids (major), ±unknowns (minor). Substrate and ecology: on bark of desert scrubs and trees (Fouquieria diguetii, Jatropha sp., Machaerocereus gummosus), coastal between 30 and 300 m or in the Sierra Laguna between 600 and 1250 m World distribution: North and South America Sonoran distribution: Baja California Sur. Notes: Usnea horrida is morphologically and chemically close to U. parvula. It differs by its reddish orange subcortical pigment, its dense to compact single layered medulla and its distributional range.