International Code of Zoological Nomenclature

This is the principle that the scientific name of a species, and not of a taxon at any other rank, is a combination of two names; the use of a trinomen for the name of a subspecies and of uninominal names for taxa above the species group is in accord with this principle.[4]

This means that in the system of nomenclature for animals, the name of a species is composed of a combination of a generic name and a specific name; together they make a "binomen".[5] No other rank can have a name composed of two names. Examples:

Species Giraffa camelopardalis

• Subspecies have a name composed of three names, a "trinomen": generic name, specific name, subspecific name:

Subspecies Giraffa camelopardalis rothschildi

• Taxa at a rank above species have a name composed of one name, a "uninominal name".

Genus Giraffa, family Giraffidae

In botanical nomenclature, the equivalent for "binominal nomenclature" is "binary nomenclature" (or sometimes "binomial nomenclature").

This is the principle that the correct formal scientific name for an animal taxon, the valid name, correct to use, is the oldest available name that applies to it.[4] It is the most important principle—the fundamental guiding precept that preserves zoological nomenclature stability. It was first formulated in 1842 by a committee appointed by the British Association to consider the rules of zoological nomenclature. Hugh Edwin Strickland wrote the committee's report.

Example:

Nunneley 1837 established Limax maculatus (Gastropoda), Wiktor 2001 classified it as a junior synonym of Limax maximus Linnaeus, 1758 from S and W Europe. Limax maximus was established first, so if Wiktor's 2001 classification is accepted, Limax maximus takes precedence over Limax maculatus and must be used for the species.

There are approximately 2-3 million cases of this kind for which this principle is applied in zoology.

The principle of coordination is that within the family group, genus group and species group, a name established for a taxon at any rank in the group is simultaneously established with the same author and date for taxa based on the same name-bearing type at other ranks in the corresponding group.[4] In other words, publishing a new zoological name automatically and simultaneously establishes all corresponding names in the relevant other ranks with the same type.

In the species-group, publishing a species name (the binomen) Giraffa camelopardalis Linnaeus, 1758 also establishes the subspecies name (the trinomen) Giraffa camelopardalis camelopardalis Linnaeus, 1758. The same applies to the name of a subspecies; this establishes the corresponding species name.

In the genus-group, similarly, publishing the name of a genus also establishes the corresponding name of a subgenus (or vice versa): genus Giraffa Linnaeus, 1758 and subgenus Giraffa (Giraffa) Linnaeus, 1758.

In the family-group, publication of the name of a family, subfamily, superfamily (or any other such rank) also establishes the names in all the other ranks in the family group (family Giraffidae, superfamily Giraffoidea, subfamily Giraffinae).

Author citations for such names (for example a subgenus) are the same as for the name actually published (for example a genus). It is immaterial if there is an actual taxon to which the automatically established name applies; if ever such a taxon is recognised, there is a name available for it.

This is the principle that in cases of conflicts between simultaneously published divergent acts, the first subsequent author can decide which has precedence. It supplements the principle of priority, which states that the first published name takes precedence. The principle of the first reviser deals with situations that cannot be resolved by priority. These items may be two or more different names for the same taxon, two or more names with the same spelling used for different taxa, two or more different spellings of a particular name, etc. In such cases, the first subsequent author who deals with the matter and chooses and publishes the decision in the required manner is the first reviser, and is to be followed.[6]

Example:

Linnaeus 1758 established Strix scandiaca and Strix noctua (Aves), for which he gave different descriptions and referred to different types, but both taxa later turned out to refer to the same species, the snowy owl. The two names are subjective synonyms. Lönnberg 1931 acted as first reviser, cited both names and selected Strix scandiaca to have precedence.

This is the principle that the name of each taxon must be unique. Consequently, a name that is a junior homonym of another name must not be used as a valid name.[4]

It means that any one animal name, in one particular spelling, may be used only once (within its group). This is usually the first-published name; any later name with the same spelling (a homonym) is barred from being used. The principles of priority and first reviser apply here. For family-group names the termination (which is rank-bound) is not taken into account.

Genera are homonyms only if exactly the same — a one-letter difference is enough to distinguish them.

Examples:

Argus Bohadsch, 1761 (Gastropoda) (was made available for homonymy by ICZN in Opinion 429, Bohadsch 1761 was non-binominal - this had the effect that no other one of the various following names Argus can be used for a taxon)

Argus Scopoli, 1763 (Lepidoptera: Lycaenidae: Polyommatinae)

Argus Scopoli, 1777 (Lepidoptera: Nymphalidae: Satyrinae)

Argus Poli, 1791 (Bivalvia)

Argus Temminck, 1807 (Aves)

Argus Lamarck, 1817 (Lepidoptera: Hesperiidae)

Argus Walckenaer, 1836 (Araneae)

Argus Gerhard, 1850 (Lepidoptera: Lycaenidae: Theclinae)

The following are not homonyms of Argus:

Argua Walker, 1863 (Lepidoptera), Argusa Kelham, 1888 (Aves), Argusina Hebard, 1927 (Dermaptera), †Arcus Hong, 1983 (Diptera), Argas Latreille, 1795 (Araneae), Argulus Müller, 1785 (Crustacea).

The following names are not homonyms of each others:

Isomya Cutler & Cutler, 1985 (Sipunculida), Isomyia Walker, 1859 (Diptera).

Adelomya Mulsant & Verreaux, 1866 (Aves), Adelomyia Bonaparte, 1854 (Aves), †Adelomys Gervais, 1853 (Mammalia), †Adolomys Shevyreva, 1989 (Mammalia), Adulomya Kuroda, 1931 (Bivalvia).

In species, there is a difference between primary and secondary homonyms. Some spelling variants are explicitly defined by the Code as being homonyms. Otherwise the one-letter difference rule applies.

Primary homonyms are those with the same genus and same species in their original combination. The difference between a primary junior homonym and a subsequent use of a name is undefined, but it is commonly accepted that if the name referred to another species or form, and if there is in addition no evidence the author knew that the name was previously used, it is considered as a junior homonym.

Examples:

Drury (1773) established Cerambyx maculatus (Coleoptera) for a species from Jamaica. Fueßlin (1775) established Cerambyx maculatus for a different species from Switzerland, and did not refer to Drury's name. Fueßlin's name is a junior primary homonym.

Scopoli (1763) established Curculio fasciatus (Coleoptera) for a species from Slovenia. Strøm (1768) established Curculio fasciatus for another species from Norway. De Geer (1775) established Curculio fasciatus for a 3rd species from Sweden. Müller (1776) established Curculio fasciatus for a 4th species from Denmark. Fourcroy (1785) established Curculio fasciatus for a 5th species from France. Olivier (1790) established Curculio fasciatus for a 6th species from France. Marsham (1802) established Curculio fasciatus for a 7th species from Britain. All these names had descriptions that clarified that different species were meant, and that their authors did not know that the name had been established by a previous author.

Secondary homonyms can be produced if taxa with the same specific name but different original genus are later classified in the same genus (Art. 57.3, 59). A secondary synonym is only a temporary state, it is only effective in this classification. If another classification is applied, the secondary homonymy may not be produced, and the involved name can be used again (Art. 59.1). A name does not become unavailable or unusable if it was once in the course of history placed in such a genus where it produced a secondary homonymy with another name. This is one of the rare cases where a zoological species does not have a stable specific name and a unique species-author-year combination, it can have two names at the same time.

Example:

Nunneley (1837) established Limax maculatus (Gastropoda), Wiktor (2001) classified it as a junior synonym of Limax (Limax) maximus Linnaeus, 1758 from S and W Europe. Kaleniczenko, 1851 established Krynickillus maculatus for a different species from Ukraine. Wiktor, 2001 classified both Limax maximus Linnaeus, 1758 and Krynickillus maculatus Kaleniczenko, 1851 in the genus Limax. This meant that L. maculatus Nunneley, 1837 and K. maculatus Kaleniczenko, 1851 were classified in the same genus, so both names were secondary homonyms in the genus Limax, and the younger name (from 1851) could not be used for the Ukrainian species. This made it necessary to look for the next younger available name that could be used for the Ukrainian species. This was Limax ecarinatus Boettger, 1881, a junior synonym of K. maculatus Kaleniczenko, 1851.

For Wiktor (2001) and those authors who follow Wiktor's system the name of the Ukrainian species must be Limax ecarinatus Boettger, 1881. For the others who classify Limacus as a separate genus, the name of the Ukrainian species must be Limacus maculatus (Kaleniczenko, 1851).

So the Ukrainian species can have two names, depending from its generic classification. Limax ecarinatus, Limacus maculatus, the same species.

Article 59.3 states that in exceptional cases, junior secondary homonyms replaced before 1961 by substitute names can become invalid, "...unless the substitute name is not in use," an exception of the exception. However, the ICZN Code does not give an example for such a case. It seems that this passage in the ICZN Code is widely ignored. It also does not define what the expression "is not in use" should mean.

Example:

Glischrus caelata Studer, 1820 (Gastropoda) was once classified in the genus Helix, and became a junior secondary homonym of Helix caelata [Vallot], 1801. Locard (1880) established a replacement name Helix glypta, which has very rarely been used. The species is now known as Trochulus caelatus (Studer, 1820), and Art. 59.3 is commonly ignored.[note 1]

Double homonymy (genus and species) is not homonymy in the strict sense: if the genera are homonyms and belong to different animal groups, the same specific names can be used in both groups.

Examples:

The name Noctua Linnaeus, 1758 was established for a lepidopteran subgenus. In 1764 he established a genus Noctua Linné ,1764 for birds, ignoring that he had already used this name a few years ago in Lepidoptera. Noctua Linné, 1764 (Aves) is a junior homonym of Noctua Linnaeus, 1758 (Lepidoptera).

Garsault (1764) used Noctua for a bird and established a name Noctua caprimulgus Garsault, 1764 (Aves). Fabricius (1775) established a name Noctua caprimulgus Fabricius, 1775 (Lepidoptera), thus creating a double homonym. Double homonymy is no homonymy, both names are available.

The same happened with Noctua variegata Jung, 1792 (Lepidoptera) and Noctua variegata Quoy & Gaimard, 1830 (Aves).

For disambiguating one genus-group name from its homonym, it is important to cite author and year. Citing the author alone is often not sufficient.

Examples:

Echidna Forster, 1777 (Actinopterygii), not Echidna Cuvier, 1797 (Mammalia)

Ansa Walker, 1858 (Lepidoptera), not Ansa Walker, 1868 (Hemiptera)

Helix balcanica Kobelt, 1876, not Helix balcanica Kobelt, 1903 (both Gastropoda)

Conus catenatus Sowerby, 1850, not Conus catenatus Sowerby, 1875 (both Gastropoda)

The name Ansa can only be used for a lepidopteran taxon. If that name cannot be used (for example because an older name established prior to 1858 takes precedence), this does not mean that the 1868 name can be used for a hemipteran genus. The only option to use the 1868 name for the hemipteran taxon is to get the 1858 name officially suppressed by the commission.

In some cases, the same genus-group or species-group name was published in the same year by the same author. In these cases it is useful to cite the page where the name was established.

Amydona Walker, 1855 (Lepidoptera: Limacodidae) (p. 1110), not Amydona Walker, 1855 (Lepidoptera: Lasiocampidae) (p. 1413)

Betousa Walker, 1865 (Lepidoptera: Thyridae) (p. 1111), not Betousa Walker, 1865 (Lepidoptera: Noctuidae) (p. 1208).

Cicada variegata Fabricius, 1775 (p. 684), not Cicada variegata Fabricius, 1775 (p. 686) (both Auchenorrhyncha).

Noctua marginata Fabricius, 1775 (p. 597), not Noctua marginata Fabricius, 1775 (p. 610) (both Lepidoptera: Noctuidae).

Clausilia (Albinaria) oertzeni Boettger, 1889 (p. 42), not Clausilia (Albinaria) schuchi var. oertzeni Boettger, 1889 (p. 52) (both Gastropoda: Clausiliidae).

There are cases where two homonyms were established by the same author in the same year on the same page:

Zonites verticillus var. graeca Kobelt, 1876 (Gastropoda) (p. 48), not Zonites albanicus var. graeca Kobelt, 1876 (p. 48).

Animal, plant, and fungi nomenclature are entirely independent from each other. The most evident shortcoming of this situation (for their use in biodiversity informatics) is that the same generic name can be used simultaneously for animals and plants. For this kind of homonym the expression "hemihomonym" is sometimes used. Far more than 1000 such names are known.[7]

Examples:

The generic name Dryas L. (1753) represents a genus of magnoliophytan plants (family Rosaceae), and at the same time Dryas Hübner, 1807 is also a lepidopteran insect genus (family Nymphalidae).

The genus Tandonia was established in animals (Gastropoda: Tandonia), in plants (Euphorbiaceae) and in Fungi (Ascomycetes).

Other examples for sometimes well known plant names with zoological equivalents are Aotus (Fabaceae and Mammalia), Arenaria (Caryophyllaeceae and Aves), Betula(Betulaceae and Hymenoptera), Chloris (Cactaceae and Aves), Dugesia (Asteraceae and Plathelminthes), Erica (Ericaceae and Araneae), Hystrix (Poaceae and Mammalia), Iris (Asparagales and Orthoptera), Liparis (Orchidaceae and Actinopterygii), Phalaenopsis (Asparagales and Aves), Pinus (Pinaceae and Mollusca), Prunella (Lamiaceae and Aves), Ricinus (Fabaceae and Acari), Taxus (Taxaceae and Mammalia), Typha (Typhaceae and Porifera), Ulva (Ulvophyceae and Lepidoptera), Viola (Violaceae and Lepidoptera).

For names above the family level, the principle of homonymy does not apply.

Examples:

Pulmonata is usually used for a very prominent group in Gastropoda, but the name is also (rarely) used for a group in Arachnida.

Reticulata is used as an order in Foraminifera, and as an undefined higher group in Ephemeroptera.

Homonyms occur relatively rarely in families (only if generic names are identical or very similar and adding an ending "-idae" produces identical results). Discovering such a homonymy usually produces the same problems as if there were no rules: conflicts between entirely independent and unconnected groups of taxonomists working in different animal groups. Very often the Commission must be asked to take a decision.

Examples:

Bulimina (Foraminifera) and Buliminus (Gastropoda) give both Buliminidae, and both families were used since the 1880s. When the homonymy was discovered 110 years later in the 1990s, the younger (gastropod) taxon had to receive a new family name, and the commission needed was asked for a solution (Opinion 2018).

Claria (Rotifera) and Clarias (Actinopterygii) give both Clariidae, but only the actinopterygian fish name was used since 1845. Shortly after Clariidae had been proposed in Rotifera in 1990, the homonymy was discovered and the commission had to decide that the Rotiferan family had to be amended to Clariaidae (Opinion 2032).

This is the principle that each nominal taxon in the family group, genus group, or species group has—actually or potentially—a name-bearing type fixed that provides the objective standard of reference that determines what the name applies to.

This means that any named taxon has a name-bearing type, which allows the objective application of that name. Any family-group name must have a type genus, any genus-group name must have a type species, and any species-group name can (not must) have one or more type specimens (holotype, lectotype, neotype, syntypes, or others), usually deposited in a museum collection. The type genus for a family-group name is simply the genus that provided the stem to which was added the ending "-idae" (for families). Example:

The family name Spheniscidae has as its type genus the genus Spheniscus Brisson, 1760.

The type species for a genus-group name is more complicated and follows exactly defined provisions in articles 67–69. Type species are very important, and no general zoological database has recorded the type species for all genera. Except in fishes and some minor groups, type species are rarely reliably recorded in online animal databases. In 60% of the cases the type species can be determined in the original publication. The type species is always the original name of the taxon (and not the currently used combination).

Example:

The correctly cited type species of Locusta Linnaeus, 1758 (Caelifera) is Gryllus migratorius Linnaeus, 1758, not Locusta migratoria (Linnaeus, 1758).

Designation and fixation have different meanings. A designation is the proposal of the type species. It is not necessary to have spelled the name of the genus or species correctly with correct authors (articles 67.2.1, 67.6, 67.7), type species are always the correctly spelled name. If the designation is valid, the type species is fixed.

A designation can also be invalid and ineffective—for example—if the genus had already a previously fixed type species, or if a type species was proposed that was not originally included, or contradicted the description or figure for a genus for which no species had originally been included.

There are various possible modes of type species designation. This is their order of legal importance, with approximate proportions of occurrence[note 2] and examples:

• Superior type fixation:

Designation by ICZN under the plenary powers (3 %)

Example:

Galba Schrank, 1803 (Gastropoda) was established with one species included, Galba pusilla Schrank, 1803. This would be the type species by monotypy. In Opinion 1896 (published in 1998) this type fixation was set aside and Buccinum truncatulum Müller, 1774 was fixed as type species under the plenary power(s) (now Galba truncatula).

Designation under Art. 70.3 (misidentified type species) (1 %)

Examples:

Bollingeria Forcart, 1940 (Gastropoda) was established with its type species Chondrus pupoides Krynicki, 1833 proposed by original designation. But Forcart 1940 misidentified the type species and meant Bulimus lamelliferus Rossmässler, 1858. It would be convenient to designate Bulimus lamelliferus as type species under Art. 70.3.

Helisoma Swainson, 1840 (Gastropoda) was established with one species included, cited by Swainson as "H. bicarinata Sow. Gen. f. 4". This suggested that the type species was misidentified, and that Planorbis campanulatus Say, 1821 and not Planorbis bicarinatus Say, 1819 was meant. But since the incorrect type species Planorbis bicarinatus has been regarded as type, it would be convenient to fix this as type under Art. 70.3.

• Type fixation in the original work:

Original designation (31 %)

Examples:

Montfort 1810 established the genus Theodoxus (Gastropoda) and designated Theodoxus lutetianus Montfort 1810 as type species (now Theodoxus fluviatilis).

Vest 1867 established the subgenus Clausilia (Isabellaria) (Gastropoda) and designated Clausilia isabellina Pfeiffer, 1842 as type species (now Isabellaria isabellina).

Riedel 1987 established the genus Turcozonites (Gastropoda) and designated Zonites wandae Riedel, 1982 as type species (now Turcozonites wandae).

Monotypy (28 %)

Examples:

Anodonta Lamarck, 1799 (Bivalvia) was originally established with one included nominal species, Mytilus cygneus Linnaeus, 1758. This is the type species fixed by monotypy (now Anodonta cygnea).

Microcondylaea Vest 1866 (Bivalvia) was originally established with two included nominal species, Unio bonellii Férussac, 1827 and with doubts Anodonta lata Rafinesque, 1820. Doubtfully included species do not count, type species is Unio bonellii fixed by monotypy (now Microcondylaea bonellii).

Absolute tautonymy (2 %)

Examples:

Kobelt 1871 established the gastropod genus-group name Candidula and included 23 species. Among these was Glischrus candidula Studer 1820. Glischrus candidula is type species fixed by absolute tautonymy (now Candidula unifasciata).

Draparnaud 1801 established the gastropod genus Succinea and included two species, Succinea amphibia Draparnaud 1801 and Succinea oblonga Draparnaud 1801. Among the synonyms of S. amphibia, Draparnaud listed a name Helix succinea Müller 1774. Synonyms do count here, so Helix succinea is type species by absolute tautonymy (now Succinea putris).

Kobelt 1904 established the gastropod subgenus Iberus (Balearica) and included 10 species. Among these was Helix balearica Rossmässler 1838, which Kobelt cited as Iberus (Balearica) balearicus. The ending -us is irrelevant here, Helix balearica is type species by absolute tautonymy (currently Iberellus balearicus or Iberellus hispanicus).

Euxinolauria Lindholm, 1924 (Gastropoda: Lauriidae) was established as a new replacement name for Caucasica Caziot & Margier, 1909 (not Caucasica Boettger, 1877 (Gastropoda: Clausiliidae)). Caucasica Caziot & Margier, 1909 contained originally four species, among which was Pupa caucasica Pfeiffer, 1857. This is the type species for Caucasica Caziot & Margier, 1909 fixed by absolute tautonymy, and also for Euxinolauria (now Euxinolauria caucasica).

The following examples do not represent absolute tautonymy: Scomber scombrus Linnaeus, 1758 (Actinopterygii), Babyrousa babyrussa (Linnaeus, 1758) (Mammalia), Suricata suricatta (Schreber, 1776) (Mammalia), Merlangius merlangus (Linnaeus, 1758) (Actinopterygii), Isabellaria isabellina (Pfeiffer, 1842) (Gastropoda), Rupestrella rupestris (Philippi, 1836) (Gastropoda).

Linnean tautonymy (0.3 %)

Example:

Linnaeus 1758 established Castor (Mammalia) and included two species, Castor fiber and Castor moschatus. Among the synonyms of Castor fiber was cited the one-word name Castor with references to six pre-Linnean works (Gesner 1598, Rondelet 1554, Jonston 1650, Dodart 1676, Ray 1693 and Aldrovandi 1649). Castor fiber Linnaeus 1758 is type species fixed by Linnean tautonymy (now Castor fiber).

• Subsequent methods of type fixation:

Subsequent monotypy (2 %)

Examples:

Valvata Müller, 1773 (Gastropoda) was established with a short description and without species. Müller 1774 included one species Valvata cristata Müller 1774. Valvata cristata is type species by subsequent monotypy (now Valvata cristata).

Omphiscola Rafinesque, 1819 (Gastropoda) was established without species included. Beck 1837 [1838] included one species Buccinum glabrum Müller, 1774. Buccinum glabrum is type species by subsequent monotypy (now Omphiscola glabra).

Subsequent absolute tautonymy (only very few cases)[note 3]

Examples:

Alosa Garsault, 1764 (Actinopterygii) was established without included species. As first author, Cuvier, 1829 included two species Clupea alosa and Clupea fincta. Type species is Clupea alosa Linnaeus 1758 by subsequent absolute tautonymy (now Alosa alosa).

Rupicapra Garsault, 1764 (Mammalia) was established without included species. As first author, Blainville, 1816 included three species Capra rupicapra Linnaeus, 1758, Capra pudu, and Capra americana. Type species is Capra rupicapra by subsequent absolute tautonymy (now Rupicapra rupicapra).

Subsequent Linnean tautonymy (only theoretical, there might be no case)

Subsequent designation (32 %)

Examples:

Aplexa Fleming, 1820 (Gastropoda) was established with two species, Bulla hypnorum Linnaeus, 1758 and Bulla rivalis Turton, 1807. Herrmannsen 1846 fixed Bulla hypnorum as type by subsequent designation (now Aplexa hypnorum).

Pseudanodonta Bourguignat 1877 (Bivalvia) was established with seven species, Anodonta complanata Rossmässler 1835, and six others. Westerlund 1902 validly designated Anodonta complanata as type species (nowPseudanodonta complanata).

A species-group name can have a name-bearing type specimen, but this is not a requirement. In many cases species-group names have no type specimens, or they are lost. In those cases the application of the species-group name is usually based on common acceptance. If there is no common acceptance, there are provisions in the Code to fix a name-bearing type specimen that is binding for users of that name. Fixing such a name-bearing type should only be done if this is taxonomically necessary (articles 74.7.3, 75.2, 75.3).

Examples:

Aptenodytes patagonica Miller, 1778 is either based on a type specimen, perhaps deposited in the Natural History Museum London or somewhere else, or its type is lost. This is now irrelevant because the usage of the name (as Aptenodytes patagonicus) for the king penguin is unambiguously accepted.

The name-bearing type for Homo sapiens Linnaeus, 1758 is deposited in Uppsala (the bones of Carl von Linné). This is a lectotype designated by Stearn 1959, correctly but unnecessarily because the usage of the name was unambiguous at that time, and still is.

In the species group gender agreement applies. The name of a species, in two parts, a binomen, say, Loxodonta africana, and of a subspecies, in three parts, a trinomen, say Canis lupus albus, is in the form of a Latin phrase, and must be grammatically correct Latin. If the second part, the specific name (or the third part, the subspecific name) is adjectival in nature, its ending must agree in gender with the name of the genus. If it is a noun, or an arbitrary combination of letters, this does not apply.

• For instance, the generic name Equus is masculine; in the name "Equus africanus," the specific name africanus is an adjective and its ending follows the gender of the generic name.
• In Equus zebra the specific name zebra is a noun, it may not be "corrected" to "Equus zebrus".
• In Equus quagga burchellii the subspecific name burchellii is a noun in the genitive case ("of Burchell").

If a species is moved, therefore, the spelling of an ending may need to change. If Gryllus migratorius is moved to the genus Locusta, it becomes Locusta migratoria. Confusion over Latin grammar has led to many incorrectly formed names appearing in print. An automated search may fail to find all the variant spellings of a given name (e.g., the spellings atra and ater may refer to the same species).