Type: USA. UTAH. Eekker’s ranch, 1828 m, on dry exposed, red sandstone, May 1951, Flowers 366 (UPS, holotype; MIN, isotype). FIG. 6. 

 ≡ Polysporina arenacea (H. Magn.) K. Knudsen & Kocourk., Mycotaxon 105: 157 (2008), partially misapplied to Sarcogyne paradoxa Kocourk. & K. Knudsen.

Description. Hypothallus endolithic, IKI-, continuous with attaching hyphae. Thallus of dispersed broadly attached areoles, up to 1 mm high and 1.0–1.5 mm diameter, can cover areas up to 5 cm, sometimes replicating by division and forming pulvinate clusters. Upper surface white, appearing pruinose but epruinose, white pigmentation not translucent in water, without fissures or sliced by deep vertical fissures forming a radial pattern around the disc, sometimes bisected  by horizontal fissures, eventually becoming deeply cross-hatched, the combination of white pigmented surfaces and dark brown fissures giving the areoles a distinctive white/brown pattern. Epicortex variable, up to 100 µm thick, formed of intricate hyphae 2–3 µm thick and round cells to 6 µm in diam. in gelatinized polysaccharide matrix, with upper surface glazed white. Cortex 50–70 µm thick, formed of anti-clinal hyphae 2–3 µm thick, with apices to 6 µm wide in gel caps forming an upper reddish-brown pigmented layer above a hyaline lower layer. Algal layer thick and even, 70–100 µm thick, continuous beneath apothecia, algal cells mostly 8–12 µm. Medulla up to 700 µm thick of hyphae 1–4 µm in diam., thin-walled and branching, IKI-, anti-clinal, mixed with abundant granules.
Apothecia immersed, one per areole, disc reddish brown to black, round, 0.2–0.6 mm wide, epruinose, very rough, formed of the apices of paraphyses in a coherent layer of epihymenial pigment. Parathecium of thin hyphae, 45–100 µm wide, sometimes elevated above the disc. Hymenium 100–130(–150) µm tall, epihymenium reddish-brown 10–15 µm tall, hymenial gel IKI+ blue (euamyloid), paraphyses 1.0–2.0 µm wide at mid-level, branching, apices expanded up to 6 µm in pigment caps. Asci clavate, mostly 70–80 × 20–25 µm, several hundred ascospores per ascus, ascospores variable from short broadly ellipsoid 3 × 2 to ellipsoid 7 × 3 µm. Subhymenium 20–30 µm tall, IKI+ blue. Hypothecium 10 µm thick, IKI-.
Pycnidia not observed. 

Chemistry. Not producing secondary metabolites. 

Ecology and Distribution. On red and Ferron sandstone and siliceous rock in Utah and North Dakota at elevations 670–1869 m in full sun. In Nex Mexico on sandstone.

Discussion. Acarospora arenacea is similar to species of the A. strigata morphological group in having a white surface with fissures. Pruina causes the white surface in the strigata-group and it is usually translucent in water. In A. arenacea a pigment causes the white layer and is not translucent in water. A similar white pigment layer also occurs in A. bolleana (H. Magn.) K. Knudsen, J.N. Adams, Kocourk. & Y. Wang.