53.1. 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.
Note 1. Even if the earlier homonym is illegitimate, or is generally treated as a synonym on taxonomic grounds, the later homonym must be rejected.
Ex. 1. The name 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 rejected. It was renamed Thuspeinanta T. Durand (1888).
Ex. 2. The name Amblyanthera Müll. Arg. (1860) is a later homonym of the validly published Amblyanthera Blume (1849) and is therefore rejected, although Amblyanthera Blume is now considered to be a synonym of Osbeckia L. (1753).
Ex. 3. The name Torreya Arn. (1838) is a nomen conservandum and is therefore not to be rejected because of the existence of the earlier homonym Torreya Raf. (1818).
Ex. 4. Astragalus rhizanthus Boiss. (1843) is a later homonym of the validly published name Astragalus rhizanthus Royle (1835) and it is therefore rejected, as was done by Boissier who renamed it A. cariensis Boiss. (1849).
53.2. A sanctioned name is illegitimate if it is a later homonym of another sanctioned name (see also Art. 15 Note 1).
53.3. When two or more generic, specific, or infraspecific names 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.
*Ex. 5. 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).
*Ex. 6. The three generic names 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.
*Ex. 7. The names Acanthoica Lohmann (1902) and Acanthoeca W. N. Ellis (1930), both designating flagellates, are sufficiently alike to be considered homonyms (Taxon 22: 313. 1973).
*Ex. 8. Epithets so similar that they are likely to be confused if combined under the same generic or specific name: chinensis and sinensis; ceylanica and zeylanica; napaulensis, nepalensis, and nipalensis; polyanthemos and polyanthemus; macrostachys and macrostachyus; heteropus and heteropodus; poikilantha and poikilanthes; pteroides and pteroideus; trinervis and trinervius; macrocarpon and macrocarpum; trachycaulum and trachycaulon.
*Ex. 9. Names not likely to be confused: Rubia L. (1753) and Rubus L. (1753); Monochaetum (DC.) Naudin (1845) and Monochaete Döll (1875); Peponia Grev. (1863) and Peponium Engl. (1897); Iris L. (1753) and Iria (Pers.) Hedw. (1806); Desmostachys Miers (1852) and Desmostachya (Stapf) Stapf (1898); Symphyostemon Miers (1841) and Symphostemon Hiern (1900); Gerrardina Oliv. (1870) and Gerardiina Engl. (1897); Urvillea Kunth (1821) and Durvillaea Bory (1826); Peltophorus Desv. (1810; Gramineae) and Peltophorum (Vogel) Benth. (1840; Leguminosae); Senecio napaeifolius (DC.) Sch. Bip. (1845, "napeaefolius"; see Art. 60 Ex. 12) and S. napifolius MacOwan (1890; the epithets being derived respectively from Napaea and Brassica napus); Lysimachia hemsleyana Oliv. (1891) and L. hemsleyi Franch. (1895) (see, however, Rec. 23A.2); Euphorbia peplis L. (1753) and E. peplus L. (1753).
Ex. 10. Names conserved against earlier names treated as homonyms (see App. IIIA): Lyngbya Gomont (vs. Lyngbyea Sommerf.); Columellia Ruiz & Pav. (vs. Columella Lour.), both commemorating Columella, the Roman writer on agriculture; Cephalotus Labill. (vs. Cephalotos Adans.); Simarouba Aubl. (vs. Simaruba Boehm.).
53.4. When it is doubtful whether names 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 or committees for the appropriate taxonomic group or groups. A recommendation may then be put forward to an International Botanical Congress, and, if ratified, will become a binding decision.
Ex. 11. Names ruled as likely to be confused, and therefore to be treated as homonyms: Ficus gomelleira Kunth (1847) and F. gameleira Standl. (1937) (Taxon 42: 111. 1993); Solanum saltiense S. Moore (1895) and S. saltense (Bitter) C. V. Morton (1944) (Taxon 42: 434. 1993); Balardia Cambess. (1829; Caryophyllaceae) and Ballardia Montrouz. (1860; Myrtaceae) (Taxon 42: 434. 1993).
Ex. 12. Names ruled as not likely to be confused: Cathayeia Ohwi (1931; Flacourtiaceae) and Cathaya Chun & Kuang (1962; fossil Pinaceae) (Taxon 36: 429. 1987); Cristella Pat. (1887; Fungi) and Christella H. Lév. (1915; Pteridophyta) (Taxon 35: 551. 1986); Coluria R. Br. (1823; Rosaceae) and Colura (Dumort.) Dumort. (1835; Hepaticae) (Taxon 42: 433. 1993); Acanthococcus Hook. f. & Harv. (1845; Rhodophyta) and Acanthococos Barb. Rodr. (1900; Palmae) (Taxon 42: 433. 1993); Rauia Nees & Mart. (1823; Rutaceae) and Rauhia Traub (1957; Amaryllidaceae) (Taxon 42: 433. 1993).
53.5. 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 treated as homonyms if they have the same epithet and are not based on the same type.
Ex. 13. The names 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 and the epithet may be repeated under Rec. 26A.1.
Ex. 14. Anagallis arvensis var. caerulea (L.) Gouan (Fl. Monsp.: 30. 1765), based on A. caerulea L. (1759), makes illegitimate the name A. arvensis subsp. caerulea Hartm. (Sv. Norsk Exc.-Fl.: 32. 1846), based on the later homonym A. caerulea Schreber (1771).
Ex. 15. 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.
Note 2. The same final epithet may be used in the names of subdivisions of different genera, and of infraspecific taxa within different species.
Ex. 16. 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.2.
53.6. When two or more homonyms have equal priority, the first of them that is adopted in an effectively published text (Art. 29, 30, 31) by an author who simultaneously rejects the other(s) is treated as having priority. Likewise, if an author in an effectively published text substitutes other names for all but one of these homonyms, the homonym for the taxon that is not renamed is treated as having priority.
Ex. 17. Linnaeus simultaneously published "10." Mimosa cinerea (Sp. Pl.: 517. 1753) and "25." M. cinerea (Sp. Pl.: 520. 1753). In 1759, he renamed species 10 M. cineraria L. and retained the name M. cinerea for species 25, so that the latter is treated as having priority over its homonym.
Ex. 18. 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.