Primary thallus: squamulose, persistent; squamules: 3-10 mm long, 1-3 mm wide; margins: entire or irregularly lobate podetia: 5-12 (35) mm tall, pale gray to glaucescent green, cup-forming; cups: 2-8 mm wide, gradually flaring, short-stalked (stalk may be shorter than cup) surface: cortex: continuous at base and persistent half the length of podetium, giving way abruptly to farinose soredia; margins: entire to irregularly dentate, proliferations absent Apothecia: very rare, long-stalked, brown ascospores: not observed Pycnidia: rare, at cup margins, subglobose conidia: not observed Spot tests: K+ yellow, changing to dingy yellow, C-, KC-, P+ brick red, UV- Secondary metabolites: two chemotypes: (1) atranorin and fumarprotocetraric acid, (2) bourgeanic acid and fumarprotocetraric acid. Habitat and ecology: on thin soil in exposed localities World distribution: Africa, Asia, Australasia, Europe, North America and South America Sonoran distribution: SE Arizona and southern California. Notes: Cladonia humilis is comparable to C. asahinae, C. chlorophaea, C. fimbriata and C. nashii, but it is different in several respects. The podetial cortex is persistent, gray and smooth, and may continue past the flaring base of the cup. The podetia of C. humilis are characteristically stout, with relatively wide cups. The characteristic presence of atranorin or bourgeanic acid distinguishes C. humilis from species other than C. nashii, which has hardly any cortex on the podetial stalk. The bourgeanic acid chemotype is sometimes recognized as a distinct species (C. conista), but at least in the Sonoran area it does not seem to be distinguishable morphologically, and it is therefore included in C. humilis. The atranorin chemotype seems to be more frequent in the Sonoran region, but in Arizona only the bourgeanic acid chemotype was recorded.