Article 33. Subsequent spellings
A subsequent spelling of a name, if different from the original spelling [Art. 32.1], is either an emendation [Art. 33.2], or an incorrect subsequent spelling [Art. 33.3], or a mandatory change [Art. 34].
Any demonstrably intentional change in the original spelling of a name other than a mandatory change is an "emendation", except as provided in Article 33.4.
33.2.1. A change in the original spelling of a name is only to be interpreted as "demonstrably intentional" when in the work itself, or in an author's (or publisher's) corrigenda, there is an explicit statement of intention, or when both the original and the changed spelling are cited and the latter is adopted in place of the former, or when two or more names in the same work are treated in a similar way.
33.2.2. The correction of an incorrect original spelling in accordance with Article 32.5 is a "justified emendation", and the name thus corrected retains the authorship and date of the original spelling [Art. 19.2].
33.2.3. Any other emendation is an "unjustified emendation"; the name thus emended is available and it has its own author and date and is a junior objective synonym of the name in its original spelling; it enters into homonymy and can be used as a substitute name, but
18.104.22.168. when an unjustified emendation is in prevailing usage and is attributed to the original author and date it is deemed to be a justified emendation.
Example. Because Helophorus, an unjustified emendation by Illiger (1801) of Elophorus Fabricius, 1775, is in prevailing use in the Coleoptera and attributed to Fabricius, it is deemed to be a justified emendation; the name Helophorus Fabricius, 1775 is to be maintained as the correct spelling.
Any subsequent spelling of a name different from the correct original spelling, other than a mandatory change or an emendation, is an "incorrect subsequent spelling"; it is not an available name and, like an incorrect original spelling [Art. 32.4], it does not enter into homonymy and cannot be used as a substitute name, but
33.3.1. when an incorrect subsequent spelling is in prevailing usage and is attributed to the publication of the original spelling, the subsequent spelling and attribution are to be preserved and the spelling is deemed to be a correct original spelling.
Example. The specific name in Trypanosoma brucii Plummer & Bradford, 1899 is in prevailing usage but is spelled brucei; brucei is deemed to be correct and its use is to be maintained.
The use of the genitive ending -i in a subsequent spelling of a species-group name that is a genitive based upon a personal name in which the correct original spelling ends with -ii, or vice versa, is deemed to be an incorrect subsequent spelling, even if the change in spelling is deliberate; the same rule applies to the endings -ae and -iae, -orum and -iorum, and -arum and -iarum.
Example. The subsequent use by Waterhouse of the spelling bennettii for the name established as Macropus bennetti Waterhouse, 1837 does not make the subsequent spelling an available name even if the act was intentional.
In any case of doubt whether a different subsequent spelling is an emendation or an incorrect subsequent spelling, it is to be treated as an incorrect subsequent spelling (and therefore unavailable), and not as an emendation.