Tulostoma niveum  

Basidiomata 4.5-12 mm diam., stipitate at maturity, subglobose or ovoid, strongly papillate with a single ostiole (peristome), the base flattened, with a slight collar around the stipe apex. Stipe to 25 mm in length and 1-2.5 mm diam., white or yellowish, smooth or striate, deeply immersed in moss, arising from copious white mycelium. Peridium composed of a membranous white outer layer, and a white to yellowish inner layer that is initially slightly velvety but becoming smooth. Ostiole obtusely conical, white, the aperture circular. Gleba pale brown or fawn.
Capillitial hyphae 2-6 µm diam., hyaline, branched, usually thick-walled with often narrow and sometimes discontinuous lumina, septate, the septa not or slightly enlarged, often yellowish, slightly constricted, 5-7 µm diam., the surface smooth or slightly and irregularly encrusted. Basidia not seen. Basidiospores 5-6 (-6.5) µm diam. excluding ornament, 5.5-7.5 µm diam. including ornament, globose or subglobose, pale yellowish, verruculose, the verrucules somewhat irregular in size and form, often coarse, mostly isolated, 0.2-0.6 µm high and 0. 2-1.6 µm diam.

Map of known distribution in Great Britain and Ireland.