Aspicilia calcarea

in an irregular manner, not unlike crazy-paving. (See images).
It usually forms thin layers on rocks, but may also form thick encrustaceans. Towards the edge of the thallus the cracks die out.
The margin or prothallus may be white, else it is a blush green colour where the underlying photobiont is showing through. This is unusual as the prothallus is defined as lacking the photobiont (see New Zealand's Lichens, Malcolm & Galloway). If the thallus is lightly scraped the underlying green photobiont is seen.
The reproductive structures or apothecia are very distinctive having the following characteristics: (i) the are black and sunken into the thallus (ii) their shape varies from circular to elongate, often merging.
(iii) there is often more than one apothecia per areole (paving slab).

Distribution & Ecology
Frequently found on hard limestone. It is common on rocks in low lying unexposed areas; it is a typical lichen on limestone grave stones and memorials. It grows locally profusely on the top of limestone walls where it is regularly wetted by the rain. Rarely found on sandstone, basalt or granite or on Man made materials such as mortar and cement. It is not surprising that this lichen is widespread throughout Ireland because of the prominence of limestone and granite in the Irish landscape. Its distribution maps the drainage basin of the river Shannon.

Comments
Its general absence from cement and mortar walls is conspicuous. A.calcarea's photosynthetic partner is a unicellular green cyanobacteria, a chlorococcoid. This can be seen if the thallus is scraped. It may be confused with some Lecanora species and Rhizocarpon calcareum (not recorded in Ireland).

Other Names
None

Other Aspicilia Found in Ireland

Aspicilia caesiocinerea (Nyl. ex Malbr.) Arnold
Aspicilia cinerea (L.) Korb.
Aspicilia contorta subsp. contorta
Aspicilia grisea Arnold
Aspicilia intermutans (Nyl.) Arnold
Aspicilia laevata (Ach.) Arnold
Aspicilia leprosescens (Sandst.) Hav.
Aspicilia recedens (Taylor) Arnold
Aspicilia subdepressa auct. brit., non (Nyl.)Arnold

Text & Images © Paul Whelan, 2007, www.biology.ie