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So what is so distinctive about the cracked (areolate) cortex of this crustose species of siliceous rocks? Well, it is all in the medulla layer of the thallus, which is usually white but turns blue with the addition of iodine. Otherwise the grey-brown to brown-red thallus, with a black prothallus is very like many other similarly-sized members of the genus. But the combination of the following characters together with the reaction of the medulla in iodine makes it stand out: K+ red-purple epithecium (top part of disc in section), eight-spored ascus and the three-septate to submuriform, colourless spores turning green-brown with age. In Ireland, it grows on rocks and walls but is rarely recorded.
Original text submitted by Vince J. Giavarini