Japewiella tavaresiana (H. Magn.) Printzen
Biatora mutabilis ss. auct. brit.
Lecidea tavaresiana H. Magn.
Thallus crustose, superficial, thin, whitish, grey-whitish, pale greenish-grey, sometimes with a blackish hypothallus. Apothecia usually numerous, as if resting on the thallus, 0.3-0.6 mm in diameter, disc flat, reddish, reddish-brown, becoming blackish-brown at the end, margin thick at first then thinner, persistent, concolorous. Asci 8-spored, utriform, 55-65 x 15-20 (25) µm. Colorless, broadly elliptical spores, typically thick-walled, 13-17 x 8.5-10 µm according to literature, 12-16 x 7-9 µm according to our measurements. Photobiont: chlorococcoid. C-, K- or sometimes K+/- yellow, KC-, P-, UV-. Rare species growing on the branches or on the trunks of shrubs or small trees (Salix,...) in very humid and dark places that are difficult to access, often with Lecanora jamesii.
N.B. Species often confused with Lecidea erythrophaea but the latter species has smaller spores without thick walls. Also easy to confuse with Bacidia but the latter species have very different spores.