Diagnosis. Differs from P. dilatatum by the presence of caperatic instead of echinocarpic acid; currently known only from
the Galapagos.

Type: Ecuador. Galápagos: Isla Floreana, trail from Black Beach to highlands, transition zone, 25-Apr-1976, Weber, W.A. s.n. (L-62899, COLO 294622 – holotype! originally selected by M.E. Hale).

Description. Thallus saxicolous; upper surface whitish gray to bright white, shiny, epruinose, emaculate, not cracked; densely lobulate, lobules breaking apart into cauliflower-like soralia with granular coarse soredia; lobes small to moderate-sized, 2–7 mm wide, ± rotund, eciliate, distinctly delimited along the lobe edge by a conspicuous black rim; lower surface with a ± narrow, deep brown erhizinate, ~1–2.5 mm wide margin, blackening and rhizinate towards the thallus center; rhizines short, stout, black, mostly simple, rarely sparsely branched; medulla white. Apothecia very rarely observed (on a few, non-sorediate specimens only, cited separately below), 0.3–7 mm in diam., with a cupulate thalline margin, disc dark brown, deeply concave, imperforate; asci clavate, Lecanora-type, ascospores 8/ascus, simple, broadly ellipsoid, (9.9–)10.6–12.8(–12.9) × (5.9–)6.5–7.9(–7.1) μm (n = 25), thick walled (~ 1.5 μm thick). Pycnidia immersed, flask-shaped to globose; ostiole brownish black; conidia sublageniform, 6.0–7.5 × ~1.0–1.5 μm (n = 12).

Chemistry. Cortex with atranorin [P+ yellow, K+ yellow, KC–, C–, UV–]; medulla with protocetraric and caperatic acid [P+ yellow turning orange, K+ dirty yellowish brown, KC–, C–, UV–].

Etymology. Named in honor of William A. Weber, outstanding pioneer in the exploration of the Galapagos long overdue.  Mason Hale first used this name, annotating several specimens already in 1977, also selecting the holotype here designated by us. Bill Weber celebrated his 100th birthday on November 16, 2018. We wish him all the best and apologize for this now much-belated birthday gift.

Ecology and distribution. Endemic to the Galapagos; a common, exclusively saxicolous species throughout the coastal, dry, and transition zones; both at sunny and exposed and at ± shaded and sheltered sites; fertile specimens are rare.

Notes. Hale already annotated several specimens collected by Weber with this new name, apparently being aware that the chemistry does not match that of the very similar P. dilatatum. Although both substances are polyketides and both originate from the acetyl-polymalonyl pathway, they are biosynthetically nevertheless not closely related (J.A Elix, pers. com.). Therefore, we here formalize Mason Hale’s discovery of the new species. Hale at the time did not provide a formal description, but the type is clearly labeled with the name ‘Parmotrema weberi’ in his own handwriting.