Order: Arthoniales       Family: Arthoniaceae Species: Arthonia calcarea (Opegrapha calcarea) Growth type crustose, graphid. Opegrapha calcarea includes O.conferta, O.chevallieri, O.saxatalis and other former species. Very variable. Thin or immersed thallus with black, unbranched lirellae to 1.2mm. Lirellae can be star-shaped, slit-like or form piled heaps. Asci are 8-spored, spores 3-septate, 16-20 x 4-5(-6.5) µm. Microscope photograph below. Occurs on shaded calcareous or basic rocks and on man-made substrates. Also found on shaded basic to acidic coastal rocks. Thallus: chemical reactions negative. Similar: Opegrapha cesareensis. Lirellae to 2mm, spores 4-5 septate, 15-22 x 4-5 µm.

     



Microscope photograph

Arthonia calcarea (Opegrapha calcarea) on coastal rock, Creagh, County Cork. February 2009