This MO Group represents the “Xeromphalina campanella/kauffmanii complex” of Aldrovandi et al.1 — species that cannot be distinguished macroscopically from X. campanella sensu stricto. It includes at least: X. campanella, X. enigmatica, and X. kauffmanii,
X. campanella can be distinguished from X. enigmatica only by mating studies or DNA. They’re effectively identical in macroscopic features, microscopic features, chemical features, geographic location, and substrate preference.
X. kauffmanii is an eastern N. Amer. endemic, and has a distinct substrate — hardwood — and smaller spores (3–6 x 2–3 µm).
1 Matthew S. P. Aldrovandi, James E. Johnson, Brian O’Meara, Ronald H.
Petersen & Karen W. Hughes (2015) The Xeromphalinacampanella/kauffmanii complex: species delineation and biogeographical patterns of speciation, Mycologia, 107:6, 1270-1284 https://www.tandfonline.com/doi/full/10.3852/15-087.