A name of a family, genus, or species, unless conserved (Art. 14) or sanctioned (Art. 15), is illegitimate if it is a later homonym, that is, if it is spelled exactly like a name based on a different type that was previously and validly published for a taxon of the same rank (see also Art. 53.2 and 53.4).
Tapeinanthus Boiss. ex Benth. (1848), given to a genus of Labiatae, is a later homonym of Tapeinanthus Herb. (1837), a name previously and validly published for a genus of Amaryllidaceae. Tapeinanthus Boiss. ex Benth. is therefore illegitimate and unavailable for use; it was renamed Thuspeinanta T. Durand (1888).
Torreya Arn. (1838) is a nomen conservandum and is therefore available for use in spite of the existence of the earlier homonym Torreya Raf. (1818).
Astragalus rhizanthus Boiss. (1843) is a later homonym of the validly published name A. rhizanthus Royle (1835) and is therefore illegitimate; it was renamed A. cariensis Boiss. (1849).
Molina racemosa Ruiz & Pav. (1798) (Compositae) is an illegitimate later homonym of Molina racemosa Cav. (1790) (Malpighiaceae).
Moreae Britton & Rose (in N. Amer. Fl. 23: 201, 217. 1930), based on Mora Benth. (1839), although a later homonym of Moreae Dumort. (Anal. Fam. Pl.: 17. 1829), based on Morus L. (1754), is not illegitimate as the provisions on homonymy do not apply to subdivisions of families.
A validly published earlier homonym, even if illegitimate or otherwise generally treated as a synonym, causes rejection of any later homonym that is not conserved or sanctioned (but see Art. 53.2).
Zingiber truncatum S. Q. Tong (1987) is illegitimate, being a later homonym of the validly published Z. truncatum Stokes (1812), even though the latter name is itself illegitimate under Art. 52.1; Z. truncatum S. Q. Tong was renamed Z. neotruncatum T. L. Wu & al. (2000).
Amblyanthera Müll. Arg. (1860) is a later homonym of the validly published Amblyanthera Blume (1849) and is therefore illegitimate, although Amblyanthera Blume is now considered to be a synonym of Osbeckia L. (1753).
A sanctioned name is illegitimate if it is a later homonym of another sanctioned name (see also Art. 15 Note 1).
When two or more names of genera or species based on different types are so similar that they are likely to be confused (because they are applied to related taxa or for any other reason) they are to be treated as homonyms (see also Art. 61.5). If established practice has been to treat two similar names as homonyms, this practice is to be continued if it is in the interest of nomenclatural stability.
Names treated as homonyms: Asterostemma Decne. (1838) and Astrostemma Benth. (1880); Pleuropetalum Hook. f. (1846) and Pleuripetalum T. Durand (1888); Eschweilera DC. (1828) and Eschweileria Boerl. (1887); Skytanthus Meyen (1834) and Scytanthus Hook. (1844).
Bradlea Adans. (1763), Bradleja Banks ex Gaertn. (1790), and Braddleya Vell. (1827), all commemorating Richard Bradley, are treated as homonyms because only one can be used without serious risk of confusion.
Acanthoica Lohmann (1902) and Acanthoeca W. N. Ellis (1930), both designating flagellates, are sufficiently alike to be considered homonyms (Taxon 22: 313. 1973).
Epithets so similar that they are likely to be confused if combined under the same name of a genus or species: ceylanicus and zeylanicus;chinensis and sinensis;heteropodus and heteropus;macrocarpon and macrocarpum;macrostachys and macrostachyus;napaulensis,nepalensis, and nipalensis;poikilantha and poikilanthes;polyanthemos and polyanthemus;pteroides and pteroideus;thibetanus and tibetanus; thibetensis and tibetensis;thibeticus and tibeticus; trachycaulon and trachycaulum;trinervis and trinervius.
Names not likely to be confused: Desmostachys Miers (1852) and Desmostachya (Stapf) Stapf (1898); Euphorbia peplis L. (1753) and E. peplus L. (1753); Gerrardina Oliv. (1870) and Gerardiina Engl. (1897); Iris L. (1753) and Iria (Pers.) Hedw. (1806); Lysimachia hemsleyana Oliv. (1891) and L. hemsleyi Franch. (1895) (see, however, Rec. 23A.2); Monochaetum (DC.) Naudin (1845) and Monochaete Döll (1875); Peltophorus Desv. (1810; Gramineae) and Peltophorum (Vogel) Benth. (1840; Leguminosae); Peponia Grev. (1863) and Peponium Engl. (1897); Rubia L. (1753) and Rubus L. (1753); Senecio napaeifolius (DC.) Sch. Bip. (1845, ‘napeaefolius’; see Art. 60 Ex. 21) and S. napifolius MacOwan (1890; the epithets being derived, respectively, from Napaea and Brassica napus); Symphyostemon Miers (1841) and Symphostemon Hiern (1900); Urvillea Kunth (1821) and Durvillaea Bory (1826).
Names conserved against earlier names treated as homonyms (see App. III): Cephalotus Labill. (vs Cephalotos Adans.); Columellia Ruiz & Pav. (vs Columella Lour.), both commemorating Columella, the Roman writer on agriculture; Lyngbya Gomont (vs Lyngbyea Sommerf.); Simarouba Aubl. (vs Simaruba Boehm.).
Gilmania Coville (1936) was published as a replacement name for Phyllogonum Coville (1893) because the author considered the latter to be a later homonym of Phyllogonium Bridel (1827). Treating them as homonyms has become accepted, e.g. in Index Nominum Genericorum, and the name Gilmania has been accepted as legitimate ever since. Therefore the names Phyllogonum and Phyllogonium are to continue to be treated as homonyms.
The names of two subdivisions of the same genus, or of two infraspecific taxa within the same species, even if they are of different rank, are homonyms if they are not based on the same type and have the same final epithet, or are treated as homonyms if they have a confusingly similar final epithet. The later name is illegitimate.
Andropogon sorghum subsp. halepensis (L.) Hack. and A. sorghum var. halepensis (L.) Hack. (in Candolle & Candolle, Monogr. Phan. 6: 502. 1889) are legitimate since both have the same type (see also Rec. 26A.1).
Anagallis arvensis subsp. caerulea Hartm. (Sv. Norsk Exc.-Fl.: 32. 1846), based on the later homonym A. caerulea Schreb. (1771), is illegitimate because it is itself a later homonym of A. arvensis var. caerulea (L.) Gouan (Fl. Monsp.: 30. 1765), based on A. caerulea L. (1759).
Scenedesmus armatus var. brevicaudatus (Hortob.) Pankow (in Arch. Protistenk. 132: 153. 1986), based on S. carinatus var. brevicaudatus Hortob. (in Acta Bot. Acad. Sci. Hung. 26: 318. 1981), is a later homonym of S. armatus f. brevicaudatus L. S. Péterfi (in Stud. Cercet. Biol. (Bucharest), Ser. Biol. Veg. 15: 25. 1963) even though the two names apply to taxa of different infraspecific rank. Scenedesmus armatus var. brevicaudatus (L. S. Péterfi) E. H. Hegew. (in Arch. Hydrobiol. Suppl. 60: 393. 1982), however, is not a later homonym since it is based on the same type as S. armatus f. brevicaudatus L. S. Péterfi.
The same final epithet may be used in the names of subdivisions of different genera and in the names of infraspecific taxa within different species.
Verbascum sect. Aulacosperma Murb. (Monogr. Verbascum: 34, 593. 1933) is permissible, although there is an earlier Celsia sect. Aulacospermae Murb. (Monogr. Celsia: 34, 56. 1926). This, however, is not an example to be followed, since it is contrary to Rec. 21B.3.
When it is doubtful whether names or their epithets are sufficiently alike to be confused, a request for a decision may be submitted to the General Committee (see Div. III), which will refer it for examination to the committee(s) for the appropriate taxonomic group(s). A recommendation, whether or not to treat the names concerned as homonyms, may then be put forward to an International Botanical Congress and, if ratified, will become a binding decision. These binding decisions are listed in App. VIII.
When two or more homonyms have equal priority, the first of them that is adopted in an effectively published text (Art. 29–31) by an author who simultaneously rejects the other(s) is treated as having priority. Likewise, if an author in an effectively published text replaces with other names all but one of these homonyms, the homonym for the taxon that is not renamed is treated as having priority (see also Rec. 42A.2).
Linnaeus simultaneously published “10.” Mimosacinerea (Sp. Pl.: 517. 1753) and “25.” M. cinerea (Sp. Pl.: 520. 1753). In 1759, he renamed species 10 as M. cineraria L. and retained the name M. cinerea for species 25, so that the latter is treated as having priority over its homonym.
Rouy & Foucaud (Fl. France 2: 30. 1895) published the name Erysimum hieraciifolium var. longisiliquum, with two different types, for two different taxa under different subspecies. Only one of these names can be maintained.
A homonym renamed or rejected under Art. 53.6 remains legitimate and has priority over a later synonym of the same rank should it be transferred to another genus or species.
Mimosa cineraria L. (1759), based on M. cinerea L. (Sp. Pl.: 517 [non 520]. 1753; see Art. 53 Ex. 19), was transferred to Prosopis by Druce (1914) as P. cineraria (L.) Druce. However, the correct name in Prosopis would have been a combination based on M. cinerea had not that name been successfully proposed for rejection (see App. V).
