Alder Goblet (Ciboria caucus)
Found solitary or in small groups on the fallen male catkins of the Common Alder Alnus glutinosa or willows, C. caucus is a widespread species though one that is easily overlooked due to its small size and wet habitat.
Phillips, R. (2006). Mushrooms. London: Pan Macmillan, p. 369, fig. f.
Sterry, P. and Hughes. B. (2009). Collins Complete Guide to British Mushrooms and Toadstools. London: HarperCollins, p. 326, fig. p. 327.