Under the zoological code, homonymy can only occur
within each of the three nomenclatural ranks (family-rank, genus-rank, and species-rank) but not
between them; there are thousands of cases where a species epithet is identical to a genus name but not a homonym (sometimes even occurring in the genus it is identical to, such as
Gorilla gorilla, termed a "
tautonym"), and there are some rare cases where a family-rank name and a genus-rank name are identical (e.g., the superfamily name
Ranoidea and the genus name
Ranoidea are not homonyms).
Unavailable names do not compete for homonymy. The botanical code is generally similar, but prohibits tautonyms.
Parahomonyms Under the botanical code, names that are similar enough that they are likely to be confused are also considered to be homonymous (article 53.3). For example,
Astrostemma Benth. (1880) is an illegitimate homonym of
Asterostemma Decne. (1838). The zoological code considers even a single letter difference to be sufficient to render family-rank and genus-rank names distinct (Article 56.2), though for species names, the ICZN specifies a number of spelling variations (Article 58) that are considered to be identical.
Hemihomonyms Both codes only consider taxa that are in their respective scope (animals for the ICZN; primarily plants for the ICN). Therefore, if an animal taxon has the same name as a plant taxon, both names are valid. Such names are called
hemihomonyms. For example, the name
Erica has been given to both a genus of spiders,
Erica Peckham & Peckham, 1892, and to a genus of heaths,
Erica L. Another example is
Cyanea, applied to the lion's mane jellyfish
Cyanea Péron and Lesueur and to the Hawaiian lobelioid
Cyanea Gaudich. Hemihomonyms are possible at the species level as well, with organisms in different kingdoms sharing the same
binomial nomenclature. For instance,
Orestias elegans denotes both a species of fish (kingdom
Animalia) and a species of orchid (kingdom
Plantae). Such duplication of binomials occurs in at least nine instances. ==See also==