=Singular.= =Plural.=
radius, radii.
beau, beaux.
analysis, analyses.
index, indices.
axis, axes.
basis, bases.
seraph, seraphim.
memorandum, memoranda.
phenomenon, phenomena.
crisis, crises.
erratum, errata.
stratum, strata.
oasis, oases.
cherub, cherubim.
8. Some compound nouns make the principal word plural, and some make
both words plural; as, _son-in-law_, _sons-in-law_; _man-servant_,
_men-servants_.