Rhizocarpon geographicum (L.) DC.
Yellow map lichen, world map lichen
This is one of several map lichens, so-called because at close range, each tiny lichen is bordered by black spores that resemble borders on a map. Although it can withstand two weeks of exposure to space, it is sensitive to pollution, and fares best in low pollution environments.
Identification: The pale yellow patches with black spore-lined edges do not closely resemble other lichens. Yellow map lichen is crustose, forming a thin, tightly attached surface on rocks.
Habitats: Yellow map lichen is common in cool forests, such as the Sierra Nevada mountains, northern Maine, Canada, Greenland, Iceland, and Siberia. Closer to the equator, it is found at progressively higher elevations.
Online References:
References:
9/19/2009 · Acadia National Park, Bar Harbor, Maine · ≈ 7 × 4½″ (17 × 11 cm) 
ID is uncertain 
9/28/2013 · Cadillac Mountain, Acadia National Park, Bar Harbor, Maine · ≈ 7 × 4½″ (18 × 12 cm) 
9/19/2009 · Acadia National Park, Bar Harbor, Maine · ≈ 1½ × 1′ (52 × 34 cm) ID is uncertain 
9/15/2016 · Cadillac Mountain Summit, Acadia, Bar Harbor, Maine 
9/19/2009 · Acadia National Park, Bar Harbor, Maine · ≈ 11 × 7″ (27 × 18 cm) ID is uncertain 
The gray patches are single-spored map lichen (Rhizocarpon disporum) · 9/19/2009 · Acadia National Park, Bar Harbor, Maine · ≈ 11 × 7″ (27 × 18 cm) ID is uncertain 
9/19/2009 · Acadia National Park, Bar Harbor, Maine · ≈ 5 × 8″ (13 × 19 cm) ID is uncertain 
9/19/2009 · Acadia National Park, Bar Harbor, Maine · ≈ 7 × 4½″ (17 × 11 cm) 
9/28/2013 · Cadillac Mountain, Acadia National Park, Bar Harbor, Maine · ≈ 4 × 3½″ (10 × 9 cm) 
| Kingdom | Fungi | Mushrooms, lichens, gel fungi, yeasts, and molds; also called eukaryotes |
| Division | Ascomycota | Sac fungi |
| Subdivision | Pezizomycotina | Sac fungi with filaments |
| Class | Lecanoromycetes | “Lichenized” fungi |
| Order | Lecanorales | Mostly lichen-forming fungi |
| Suborder | Lecanorineae | Lichens |
| Family | Rhizocarpaceae | |
| Genus | Rhizocarpon | |
| Species | geographicum |
Rhizocarpon geographicum description by Thomas H. Kent, not updated.
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