FOREIGN PLURAL OF NOUNS

SINGULAR ENDING (SE) PLURAL ENDING (PE)

SE - us PE - i E.G.stimulus - stimuli, radius - radii
SE - a PE- ae E.G.larva - larvae, vertebra - vertebrae
SE - um PE - a E.G. memorandum - memoranda, stratum - strata
SE - is PE - es E.G. crisis - crises, parenthesis - parentheses
SE - on PE - a E.G. criterion - criteria, phenomenon - phenomena
SE - ex, - ix PE - ices E.G. vortex - vortices, matrix - matrices
SE - eau PE - eaux E.G. bureau - bureaux, plateau - plateaux

There is a tendency for foreign word adopted in English to develop regular plural forms. Thus dictionaries now also give memorandums, criterions, bureaus.