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Porpidia rugosa (Taylor) Coppins & Fryday


A crustose lichen with a thick, white to blue-grey, smooth or crumbling to hummocky thallus, generally with small, scattered, white to greyish soralia, which are said often to form lines along cracks in the thallus (not readily visible in the photographs below); apothecia with thick, black rims and black or grey-pruinose discs, rare. A widespread but undistinguished lichen on damp, siliceous rocks, mostly in the north and west.

Refs: Smith et al. (2009), 747; Purvis et al. (1992), 497 (as P. glaucophaea); Dobson (2011), 366; Wirth (1995), 2: 766, 770 (photo, as P. glaucophaea); Wirth, Hauck & Schulz (2013), 2: 921, 925 (photo).

Host to Arthonia amylospora, which forms small, black apothecia, immersed in the thallus surface.

Porpidia 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. Dobson (2011), pp. 236-237, gives an invaluable analysis of the component genera.

 
Porpidia rugosa
On sheltered face of seacliff, Bàgh a Ghallanaich, Isle of Muck, June 2012, BLS meeting
 
Porpidia rugosa with Arthonia amylospora
Thallus with apothecia of Arthonia amylospora, on sheltered face of seacliff, Bàgh a Ghallanaich, Isle of Muck, June 2012, BLS meeting


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Uploaded February 2013, updated September 2013