Thallus: leprose, diffuse, without lobes, granules contiguous or dispersed, loose to partially fused in distinct columns or curds, 100-300 µm (-1 mm) thick upper surface: turquoise to dull blue-green or green, uneven soredia: corticate granules 30-50 µm diam., without projecting hyphae, with colorless cortex of 1-2 layers of paraplectenchymatous hyphae, globose to ellipsoid, 2-3(-7) µm in diam., surrounding an algal core medulla: lacking lower surface: occasional hyaline, with leptodermatous rhizoids arising from the cortex of the granules; rhizoids: 20-30 µm wide, lacing through granules and upper mineral particles of soil or rock; thalli forming a chasmolithic pseudomedullary layer of gelatinized rhizines, mixed with necral matter, silicate crystals, bacteria, cyanobacteria, and occasionally associated fungi, visible in dissecting microscope as a white area below the granules Spot tests: P- or P+ orange or orange-red, or dull red color below upper layer of granules Secondary metabolites: argopsin and norargopsin. Substrate and ecology: on soil and rocks, and over mosses and other lichens, esp. as pioneer on verticals exposed to rain on north or shaded slopes World and Sonoran distribution: southern California (common in the Santa Monica Mountains and in Upper Weir Canyon in Santa Ana Mountains, and associated with Torrey Pine forests on Santa Rosa Island and coastal San Diego County). Notes: Argosin and norargopsin are found in one other species of Lepraria namely, Lepraria coriensis (Hue) Sip-man (Elix 2006). However, that taxon is quite distinct from Lepraria santamonicae as it is green in color and contains usnic acid and zeorin as major components, has a distinct medulla and a granular upper surface with lobed margins, with the lobes up to 2 mm wide.