Notes on the nomenclature and taxonomy of Chiastophyllum and Umbilicus oppositifolius (≡Cotyledon oppositifolia, ≡Chiastophyllum oppositifolium) (Crassulaceae subfam. Sempervivoideae)
. Phytotaxa 564 (2): 239–247. https://doi.org/10.11646/phytotaxa.564.2.7