Lecidea grisella (images of British lichens)

Thallus continuous, smooth, becoming a cracked-areolate, whitish-grey to pale grey-brown crust, cortical surface testing red with sodium hypochlorite; apothecia immersed, flat to slightly convex, grey-pruinose to black. Widespread and common on siliceous but nutrient-enriched rocks and stonework (as when mortar has been used with base-poor rocks or birds perch on them).

Refs: Smith et al. (2009), 511; Dobson (2005), 235-6 (photo, as L. fuscoatra); Dobson (2011), 239-40 (photo).

An old 'split' of Lecidea fuscoatra, revived at species level by Aptroot et al. in Smith et al. (2009). True L. fuscoatra (page pending) is areolate from the start, with more convex areoles and apparently is more often brownish. Wirth (1995), 1: 513, gives a good photograph. L. grisella is considered the more common and widespread of the two. Lecanora rupicola, when its apothecia are parasitised by Arthonia varians, can resemble this species.

Lecidea is, obviously, 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. Dobson (2011), pp. 236-7, gives an invaluable analysis of the component genera.

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Images of British Lichens