• Rhizocarpon petraeum (Wulfen) A.Massal.
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Rhizocarpon petraeum
© Mike Simms
Rhizocarpon petraeum
© Mike Simms
Rhizocarpon petraeum
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This ice-white to chalk-white crustose species with smooth thallus and black, non-frosted discs arranged in almost concentric rings outwards from the centre occurs on slightly basic, slightly acid rocks, a habitat referred to by lichenologists as ‘intermediate’. It has a scattered distribution in Ireland, both in urban and rural areas.

Key characteristics

  • The colour of the thallus and concentric arrangement of the black discs should be sufficient to alert one to the possibility of this species
  • The similar Rhizocarpon umbilicatum, which also occurs in Ireland, has randomly distributed discs, a snowy-white thallus and is only found on quality limestones, never intermediate rocks.

Original text submitted by Simon Davey

 Simms, M. J., (2016). Rhizocarpon petraeum (Wulfen) A.Massal.. [In] LichenIreland. Accessed on 2018-08-19.