Based on the DNA analysis from the Deep Hypha issue of Mycologia (Mycologia, 98(6), 2006, pp. 982–995.), Cyathus is closely related to Crucibulum (and presumably Nidularia) and together form the proposed clade/family Nidulariaceae. The interesting thing is that the sister family is Cystodermateae and together those families are a sister to the Agaricaceae.