Both authorship and spelling of the extinct giant sloth genus Megalonyx and its type species, M.jeffersonii (Mammalia, Xenarthra, Pilosa, Megalonychidae), have been inconsistent. The genus-group name has been cited with two different authorships and three dates, and it has been spelled with two different suffixes. The species-group name has been cited with four different authors and dates, and it has been spelled with two different endings. Megalonyx Jefferson, 1799 is the first valid use of the genus-group name; the correct original spelling has the -onyx suffix. The type species of Megalonyx is Megatheriumjeffersonii Desmarest, 1822; the correct original spelling has an -ii ending. A vernacular word, megalonyx, refers to species classified in the genus Megalonyx Jefferson, 1799.
Keywords: Ground sloth; Pleistocene; Quaternary; Thomas Jefferson; Virginia; West Virginia.
Loren E. Babcock.