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Crustaceous lichen. The thallus is a bright green to brownish crust, cracked and minutely but conspicuously covered by rounded warts, often with green soredia, which may dominate the thallus surface. The apothecia are brownish black, about 1mm diameter, flat with a thin 'proper' margin, more convex when damp. Fuscidea is part of the very large 'Lecidea group' of lichens – crustose species with dark or black apothecia that lack distinct 'thalline' margins, though they will usually have differentiated 'proper' margins the same colour as the disk. Microscopic examination of mature apothecia (and sadly they are often not mature) is usually necessary even to be sure of the genus, and field identification of all but a few distinctive species is risky.
On the trunk of a young chestnut
Camera Model: NIKON D300. Exposure Time: 1/60 sec.; f/32; ISO Speed Rating: 200. Focal Length: 90.0 mm. Flash fired