Irregular -en Plurals Ablaut Umlaut Plurals

encyclopedia rarely encyclopædia - encyclopedias encyclopediae is rare  Final ex or ix becomes -ices pronounced ɨsiːz, or just adds -es: index - indices or indexes matrix - matrices vertex - vertices  Final is becomes es pronounced iːz: axis - axes crisis - crises testis - testes Axes, the plural of axis, is pronounced differently from axes ฀ æksɨz, the plural of axe.  Final -ies remains unchanged: series - series species - species  Final on becomes -a: automaton - automata criterion - criteria Phenomenon - phenomena more below polyhedron - polyhedra  Final um becomes -a, or just adds -s: Table 2.2 Nouns in final um become -a, or just adds –s. addendum Addenda agendum agenda means a list of items of business at a meeting and has the plural agendas datum data Now usually treated as a singular mass noun in both informal and educated usage, but usage in scientific publications shows a strong UKUS divide. U.S. usage prefers treating data in the singular in all contexts, including serious and academic publishing. UK usage now widely accepts treating data as singular in standard English, including educated everyday usage at least in non-scientific use. forum foraforums medium media in communications and computers; now often treated as a singular mass noun mediums spiritualists, or items of medium size etc. memorandum memorandamemorandums millennium Millennia  Final us becomes -i second declension, [aɪ] or -era or -ora third declension, or just adds -es especially in fourth declension, where it would otherwise be the same as the singular: alumnus- Alumni corpus - Corpora census - Censuses focus - Foci genus - genera prospectus - prospecti plural prospectus is rare although technically correct radius - Radii syllabus - Syllabi viscus - Viscera Virus had no plural ending in Latin; the plural in English is usually viruses.