Plural form of words ending in -us

In Latin, most second declension masculine nouns in -us form their plural in -i. However, some Latin nouns ending in -us are not second declension (cf. Latin grammar). For example, third declension neuter nouns such as opus and corpus have plurals opera and corpora, and fourth declension masculine and feminine nouns such as sinus and tribus have plurals sinūs and tribūs.

Some English words derive from Latin idiosyncratically. For example, bus is a shortened form of omnibus 'for everyone', the ablative (and dative) plural of omnis, and ignoramus is a verb form, 'we do not know'. Syllabus is a Late Latin (16th c.) word, derived from a misreading of the Greek sittybos "table of contents"; since it is not a classical word, it does not have a classical plural. [1]

The English plural of virus is viruses.[2] In most speaking communities, this is non-controversial and speakers would not attempt to use the non-standard plural in -i. However, in computer enthusiast circles in the late 20th century and early 21st, the non-standard viri form (sometimes even virii) was well attested, generally in the context of computer viruses.[3] Viri is also found in some nineteenth-century sources.[4][5]

While the number of users employing these non-standard plural forms of virus was always a small percentage of the English-speaking population, the variation was notable because it coincided with the growth of the web, a medium on which users of viri were over-represented. As the distribution of Internet users shifted to be more representative of the population as a whole during the 2000s, the non-standard forms saw decline in usage. A tendency towards prescriptivism in the computer enthusiast community, combined with the growing awareness that viri and virii are not etymologically supported plural forms, also played a part.

Nonetheless, the question of what the Latin plural of virus would have been in ancient times turns out not to be straightforward, as no plural form is attested in ancient Latin literature. Furthermore, its status as a second declension neuter noun ending in -us and not of Greek origin obscures its morphology, making guesses about how it should have been declined difficult.

There are three plural forms of octopus: octopuses, octopi, and octopodes. A fourth form octopods is occasionally used by scientists for taxonomic purposes.[14]

Currently, octopuses is the most common form in the UK as well as the US; octopodes is rare, and octopi is often objected to as incorrect.[15]

The Oxford English Dictionary[16] lists octopi, octopuses and octopodes (in that order); it labels octopodes "rare", and notes that octopi derives from the mistaken assumption that octōpūs is a second declension Latin noun. (The long "ū" is not used in the 2nd declension.) Rather, it is (Latinized) Ancient Greek, from oktṓpous (ὀκτώπους, gender masculine), whose plural is oktṓpodes (ὀκτώποδες). If the word were native to Latin, it would be octōpēs ('eight-foot') and the plural octōpedēs, analogous to centipedes and mīllipedēs, as the plural form of pēs ('foot') is pedēs. In modern Greek, it is called khtapódi (χταπόδι, gender neuter), with plural form khtapódia (χταπόδια).

Chambers 21st Century Dictionary[17] and the Compact Oxford Dictionary[18] list only octopuses, although the latter notes that octopodes is "still occasionally used"; the British National Corpus has 29 instances of octopuses, 11 of octopi and 4 of octopodes. Merriam-Webster 11th Collegiate Dictionary lists octopuses and octopi, in that order; Webster's New World College Dictionary lists octopuses, octopi and octopodes (in that order).

A search on PubMed for titles or abstracts of peer-reviewed scientific papers found octopuses in 176 papers,[19] octopi in 15 papers,[20] and no records for octopodes.[21] However, octopods was found in 53 papers.[22] When expanding the search to all fields, found octopods, octopi, and octopodes all stayed the same, but octopuses increased to 1054 papers.[23] Indicating octopuses is the most preferred in the scientific record, whereas octopodes is never used.

Fowler's Modern English Usage states that "the only acceptable plural in English is octopuses," and that octopi is misconceived and octopodes pedantic.[24]

The term octopod (plural octopods or octopodes) is taken from the taxonomic order Octopoda but has no classical equivalent. The collective form octopus is usually reserved for animals consumed for food.

The situation with the word platypus is similar to that of octopus; the word is etymologically Greek despite its Latinized ending, and so pluralizing it as if it were Latin (i.e. as platypi) is sometimes ill-considered. As with octopus, importing Greek morphology into English would have platypodes as the plural, but in practice this form is hardly attested outside of discussions about pluralization. In scientific contexts, biologists often use platypus as both the singular and plural form of the word, in the tradition of sheep or fish, but laypersons and scientists alike often use the simple English plural platypuses. Different dictionaries make different recommendations.

As a word in Botanical Latin (as distinct from Classical Latin), cactus follows standard Latin rules for pluralization and becomes cacti, which has become the prevalent usage in English. Regardless, cactus is popularly used as both singular and plural, and is cited as both singular and plural.[25] Cactuses is also an acceptable plural in English.

Facetious mock-erudite plurals in -i or even -ii are sometimes found for words ending with a sound (vaguely) similar to -us. Examples are stewardi (supposed plural of stewardess) and Elvi (as a plural for Elvis imitators). The Toyota corporation has determined that their Prius model should have the plural form Prii, even though the Latin word prius has a plural priora, the Lada Priora having prior claim to that name—though the common plural is "Priuses".[26][27] Conversely, Toyota has also said that the plural of their Lexus line is Lexus.[28] The Winklevoss twins are sometimes collectively referred to as "the Winklevi".[29]

• "What is the plural of virus?". reference.com FAQ. Retrieved 2 January 2005.