Thallus: foliose, small to medium sized, 1-4 cm wide, rounded to irregular, sometimes dying from the center, thin, ±adnate, deeply lobate lobes: ±radiating, repeatedly furcate when well developed, lobules (0.3-)0.5-1 mm wide, 80-215 µm thick (moist), linear, broadened towards the tips, not swollen, entire or incised, ±contiguous, plane, convex or concave, sometimes undulate, in the thallus center sometimes dissolved into numerous isidia upper surface: smooth to isidiate, dark olive-green or black, dull, epruinose isidia: ±globose, up to 0.3 mm wide, becoming extended and ±terete, 0.3-0.75 mm long, finally densely covering the thallus center lower surface: somewhat paler than the upper surface Apothecia: unknown Pycnidia: unknown Spot tests: all negative Secondary metabolites: none detected. Substrate and ecology: on limestone (not in the Sonoran Region), calcareous sandstone or rhyolithic rock or among mosses over rocks, in shaded situations at or above 2000 m World distribution: North America (Alaska southwards) Sonoran distribution: two localities in Grand Canyon National Park and Chiricahua Mts. of Arizona. Notes: Collema subparvum is distinguished by its rather small, rounded or lacerate, ±concave to somewhat undulating lobes and by the presence finally abundant elongated isidia in the thallus center. The species sometimes resembles Collema crispum that, however, has scale-like isidia.