Collema cristatum (L.) Weber ex F.H. Wigg.
Thalli foliose, variable, often forming arcs through die-back of the central part, lobes smooth, dull olive-green to dark brown or black, concave and elongate, with raised, incised to lobulate margins, isidia typically absent (though an isidiate variant exists); apothecia usually present and often crowded. Widespread on hard limestones and on calcareous dunes.
Refs: Smith et al. (2009), 351; Purvis et al. (1992), 220; Dobson (2005), 152 (photo); Dobson (2011), 158 (photo); Jahns (1983), 182-3 (photo); Whelan (2011), 83 (photos); Nordic Lichen Flora (2007) 3: 19, 173 (photo); Stenroos et al. (2011), 188 (photo); Wirth (1995), 1: 352, 355 (photo, wet); Wirth et al. (2004), 157 (photo); Wirth, Hauck & Schulz (2013), 1: 424, 427 (photo).