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Rhizocarpon petraeum (Wulfen) A.Massal.
 
(= R. concentricum auct.)


Thallus a rather thin, matt, chalky white to pale grey, usually cracked-areolate crust; apothecia inially set into the thallus surface, becoming only slightly raised, flat, black with grey-pruinose rims, commonly arranged in concentric circles. Widespread, often common in the north and west, on base-poor or slightly base-rich rocks and stonework, often on stone buildings and drystone walls.

Refs: Smith et al. (2009), 804; Purvis et al. (1992), 535 (as R. concentricum); Dobson (2005), 389 (photo); Dobson (2011), 393 (photo); Whelan (2011), 140 (photo); Valcárcel et al. (2003), 338-9 (photo); Thomson (1997), 527, 529 (photo, as R. concentricum).

The usual (but not constant) arrangement of apothecia in concentic rings serves to separate this species from the macroscopically otherwise closely similar R. umbilicatum, whch is restricted to limestone or strongly lime-rich rocks and micoscopically has smaller ascospores. Compare also R. reductum, especially its var. cinereum. Other lichens, notably Porpidia crustulata, also can have apothecia conspicuously arranged in concentric rings.

Rhizocarpon petraeum, population
Rhizocarpon petraeum, population
Rhizocarpon petraeum, closer view, showing iron staining
Rhizocarpon petraeum, closer view
On metalliferous, upland rockface, Langdon Common, Co. Durham, June 2009, some thalli with iron oxide staining from iron in the rock
 
Rhizocarpon petraeum
Rhizocarpon petraeum
Rhizocarpon petraeum
On ruins of baryta mining wash-house, Muirshiel, Renfrewshire, August 2008


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Uploaded September 2012; last modified updated February 2014