Hemming (1967) stated:-
It is necessary to take note of the peculiar (and unsatisfactory) way in which the specific name pluto was first published. This specific name was first published in the combination Erebia pluto in a paper by F. W. Fereday published in  (Trans. N.Z. Inst. 4 : 214-218). In this paper which was no more than a popular account of his experiences as a lepidopterist in New Zealand, Fereday inserted the following brief observation : "I may also mention a black butterfly, found on the bare summits of the snowy mountains, and of which I have several specimens, taken on the range near Castle Hill Station, west of Porter's Pass, at an altitude of over 6,000 ft. . . . I believe it to be a species of Erebia, and have named it E. pluto." The use of the single adjective "black" cannot be held to constitute an "indication" for the name Erebia pluto Fereday and the citation of a type-locality also does not constitute an "indication". The name Erebia pluto Fereday is invalid as a nomen endure. Even if this name had been duly published with an "indication", it would nevertheless have been invalid, because it is a junior secondary homonym of Erebia pluto (Prunner, 1798 (Papilio pluto de Prunner, 1798)). Fereday quickly realized that his name Erebia pluto was invalid under the Law of Homonymy, and in a paper published in May 1876 (Trans. N.Z. Inst. 7 : 302-304, pl. 9) he described and figured this species under the new name Oreina (?) othello. In the preceding year (1875, Ent. mon. Mag. 12 : 10) Hewitson had described the same species under the name Erebia merula. This is therefore the oldest available name subjectively applicable to the species to which in 1872 Fereday had given the invalid name Erebia pluto.
When in 1876 Butler established the nominal genus Percnodaimon, he provided a diagnosis and stated that the genus was monobasic and added the formula "Type P. Pluto". In a following paragraph he discussed what he called "Percnodaimon Pluto Fereday = Erebia merula Hewitson". Butler went on to express the view that the name Erebia pluto Fereday, , was nomenclatorially available because that author had stated that the species was "black" and because there was "no other black Erebia in New Zealand". As has already been explained in the present note, the arguments advanced by Butler were nomenclatorially invalid. On the other hand, the name Percnodaimon pluto as published by Butler in the paper here under discussion is certainly to be regarded as having been duly published with an "indication", Butler having expressly identified it with the duly published and documented name Erebia merula Hewitson, 1855. As shown above, the nominal species [Percnodaimon] pluto was designated by Butler as the type-species of his genus Percnodaimon, that species being attributable in this connection to Butler and taking the date 1876. This name is however invalid as a secondary homonym in exactly the same way that the name Erebia pluto Fereday, 1872, would have been invalid, if in other respects it had been a nomenclatorially available name. Accordingly, the oldest available name applicable to the present species is Erebia merula Hewitson, 1875, that name representing objectively the same taxon as that represented by the nominal species Percnodaimon pluto (Fereday MS.) Butler, 1876, and in addition bearing an older name than Oreina (?) othello Fereday, 1876, a name proposed as a replacement for the invalid name Erebia pluto Fereday, , from which it differs only in that it was duly provided with a description and figure.
The higher classification used here follows Lamas (2008).
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