Cyathus is a genus of fungi in the family Nidulariaceae. Along with the genera Crucibulum, Mycocalia, Nidula and Nidularia, they are known collectively as the bird's nest fungi due to their small nest-like fruiting bodies containing egg-shaped peridioles. The genus Cyathus was monographed by mycologist Lloyd (1906), and later Brodie (1975, 1984), and their species concepts, especially those of Brodie (1975), are followed by most mycologists.
The differentiation of Cyathus species is based on observable characters, such as fruiting body shape, coverings and plications of peridia, and microscopic characteristics such as the anatomy of peridioles, and the size and shape of basidiospores. The following characters are used to help identify Cyathus species:
Microscopic characters
The following list of species is compiled from Brodie's monograph (1975) and subsequent revision (1984), as well as articles written since then describing new species or reducing others to synonymy.