(Fig. 4e)

Primary thallus persistent, of crenulate squamules, esorediate, epruinose; podetia common, grayish green to brown, with necrotic dark brown base, short to elongate, (0.8) 1–2.5 (5) cm tall; unbranched to scarcely branched; axils closed; tips blunt or more often with several closely aggregated convex, bright red apothecia, ascyphose; surface completely ecorticate with abundantly olive green to glaucescent microsquamules and granules, rarely becoming farinose sorediate; pycnidia not seen; apothecia with red jelly.

Spot tests and chemistry: P+ yellow, K+ yellow, C-, KC-, UV-; thamnolic and didymic acids (chemotype II sensu Ahti 2000, no other chemotypes observed in Galapagos).

Distribution and ecology: Know from Isabela, San Cristóbal, Santa Cruz, and Santiago Island; by far the most common red-fruited species, common and possibly restricted to the humid highlands, on a variety of substrates like soil, rock or frequently also as epiphyte, typically amongst bryophytes; both on native and introduced trees.

Notes: In Galapagos C. didyma may be confused with C. macilenta; both have generally greenish to brown green, ecorticate podetia with microsquamules, soredia and granules. However, C. macilenta is typically very densely sorediate and its podetia typically bear very few if any microsquamules. In contrast, C. didyma is typically densely microsquamulose and true ecorticate soredia are extremely rare.

Selected specimens examined: ECUADOR. GALAPAGOS: Isabela Island, Volcán Alcedo, outer SEexposed slope and crater rim, 0°27’29”S, 91°7’19”W, 1089 m, humid zone, on wood, 5 Mar 2006, Aptroot, A. 65102 (CDS 31684), upper NNW-exposed slope inside the crater, 0°27’27”S, 91°7’23”W, 1055 m, humid zone, on bark, 5 Mar 2006, Bungartz, F. 4092 (CDS. 28056), Sipman, H.J.M. L-46 (COLO 297915); Volcán Sierra Negra, 0°50’0”S, 91°10’0”W, 800 m, humid zone, on soil, 18 Apr 1990, Sánchez - Pinto, L. 5047 (B), Villamil, 150 m, dry zone, 6 July 1906, Stewart, A. 428 (336) (COLO 255412). San Cristóbal Island, 1905- 1906, Stewart 341 (MSC), Lago El Junco, 0°53’0”S, 89°28’0”W, humid zone, on soil, 1 Mar 1994, Follmann, G. 34995 (B-KOELN 60 0173603). Santa Cruz Island, vicinity of Academy Bay, on trail to La Copa (= Media Luna), 15 Feb 1964, Weber L-40271 (H, M, US, COLO 190028), near Puntudo, 0°38’41”S, 90°20’13”W, 750 m, humid zone, on soil, 27 May 2005, Aptroot, A. 63206 (CDS 29937), along the side of a little path to El Puntudo, 0°38’55”S, 90°20’4”W, 698 m, humid zone, on rock, 28 Dec 2005, Bungartz, F. 3301 (CDS 26956), El Puntudo, 0°44’33”S, 90°18’12.6”W, 694 m, zona húmeda, sobre corteza, 17 July 2007, Nugra, F. 412 (CDS 36161), summit of the island between El Puntudo and Cerro Crocker, 700 m, humid zone, 16 Apr 1976, Weber, W.A. (COLO 296977), path from Media Luna to El Puntudo, near El Puntudo, 0°39’8.59”S, 90°20’2.8”W, 684 m, humid zone, on bryophytes, 28 Oct 2010, Yánez-Ayabaca, A. 1537 (CDS 45030). Santiago Island, Munecho rock outcrop, 0°12’35”S, 90°46’57”W, 860 m, humid zone, on rock, 23 Mar 2006, Aptroot, A. 65503 (CDS 32092).