Life Habit: lichenized. Thallus: thin (c. 0.1 mm), scattered, spidery, or continuous, becoming areolate or squamulose, forming fertile squamules or verrucae, less than 0.6 mm in diam., less than 0.3 mm thick, contiguous or dispersed; surface: ochre to pale brown, even gray or white, becoming translucent in water; cortices: lacking with apparent epinecral layer or poorly formed especially around verrucae; photobiont: primary one a chlorococcoid green alga, secondary one absent; algal layer: poorly developed or cells scattered, thickening around the apothecium; medulla: continuous with attaching hyphae, obscure in water. Apothecia: singular, immersed, expanding within areoles, verrucae or squamules; disc: brown to red, darker than thallus, round, smooth, epruinose or pruinose (pruina sometimes green or pale blue or yellow); true exciple: hyaline or yellow-brown, the hyphae obscure, 10-30 µm thick; epihymenium: yellow brown, conglutinate to sometimes gelatinized in upper part, c. 15 µm thick; hymenium: hyaline, 100-150 µm tall; paraphyses: 0.5-1 µm wide, apices not expanded; subhymenium: hyaline or yellow brown, indistinct, less than 10 µm thick; hypothecium: obscure, usually yellow-brown, 10-20 µm thick; asci: tholus K/I+ blue, 80-110 x 15-17 µm, 100+-spored; ascospores: hyaline, simple, ellipsoid, 3.5-5.5 x 1.8-2 µm. Pycnidia: not observed. Secondary metabolites: none detected. Substrate and ecology: on acid or carbonaceous rocks or pebbles.
World distribution: Europe and North America; Sonoran distribution: southern California.
Notes: Myriospora heppii has been treated as an Acarospora, but it differs from Acarospora through the K/ I+blue tholus and poorly stratified thallus with an epinecral layer. The substrate may not be as important as being periodically flushed by water; a collection from southern California (Knudsen, ASU) is on granite but was collected eastern North America. Pruinosity is rarely a species-level in a drainage on a hillside. Myriospora immersa (Fink ex character in the Acarosporaceae. Myriospora heppii can J. Hedrick) R.C. Harris is treated as a synonym. The latter only be confused with reduced specimens of Acarospora name has been applied to pruinose specimens of M. heppii smaragdula, A. glaucocarpa, or A. veronensis, but these on limestone from Wisconsin, the Ozarks and throughout species have a K/I- tholus and eucortex.