Description:
Thallus: arachnoid, chalky white
Apothecial pigments: brown K+ chocolate-brown
Paraphyses: sparingly branched and anastomosing
Asci: cylindrical
Spores: 3-septate; (14-)17-21(-24) x 5.5-6 µm.
Conidia: bacilliform, 4-6 x 0.8 µm.

Distribution and Ecology:
Falkland Islands. Five collections; four from Kidney Island, one from nearby Port William. On coastal siliceous rock. A further collection from Fox Bay, West Falkland is apparently the same species but is growing in close association with Verrucaria maura.

Notes:
Similar to the Northern Hemisphere Arthonia calcarea (syn. O. calcarea) but that species has K+ green apothecial pigments and ±clavate asci. Also similar to O. diaphoriza from New Zealand and Macquarie Island, but that species (type of O. macquariensis) has a smooth, cracked areolate thallus.

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Thallus and apothecia


Ascus & thecium


Collection with V. maura


Thallus and apothecia


Thallus and apothecia


Ascus and thecium