Acarospora macrospora (images of British lichens)

Thallus consisting of interlocking, angular, brown areoles, testing negative with KOH and sodium hypochlorite (bleach); apothecia one or more per areole, darker, immersed or with distinct margins. A very local and mainly upland species on calcareous rocks, forming sheets over the rock surface. A. fuscata is very similar and much more widespread; it diagnostically tests red with sodium hypochlorite and tends to have more sunken and irregularly-shaped apothecia.
Refs: Smith et al. (2009), 129; Purvis et al. (1992), 61; Wirth (1995), 1: 104, 105 (photo).

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