Tylothallia biformigera (Leight.) P. James & H. Kilias

    Thallus very variable, more or less thick, rimose and cracked, slightly warty to very warty, greenish if wet, glaucous grey, dirty whitish a little greenish-yellow if dry, blackish hypothallus more or less visible. Apothecia isolated or in groups and then deformed, flat at first then tubercular, 0.5 - 1.5 mm in diameter, disc flat or slightly domed, olive-brown, black then bluish-black, blackish, flexuous margin, fleeting and disappearing quickly, lighter than the disc and often powdery. Asci 50-60 x 14-18 µm according to our measurements. Spores elongate ellipsoid with rounded ends, 1-septate, 10-15 x 3-5 µm according to the literature, 11-15 (16) x 3-4 (5) µm according to our measurements. Cortex and medulla: C-, K+/- yellow to orange, KC-, P+/- orange. Photobiont: Chlorococcaceae. Rare species occurring mainly on the dry faces of vertical siliceous rocks of the foreshore in the mesic supralittoral zone and more rarely on walls.

N.B. This species appears to be parasitic in its youth on other saxicolous crustose lichens, see: Tylothallia biformigera forme jeune parasite.

N.B. Not to be confused with Cliostomum tenerum that, according to literature, may have a similar aspect, but is less greenish, Pd+ red with fusiform ascospores and grows in sheltered and dry rocky places.

_ Not to be confused to very altered specimen of the more nitrophile Lecidella scabra has a more green-bluish thallus due to numerous soredia and apothecia not tuberculate.

_ Not to be confused with Lecidella asema , C+ orange.