Word Origin & History
occasional plural suffix of words ending in -a, most of which, in Eng., are from L. nom. fem. sing. nouns, which in L. form their plurals in -ae. But plurals in -s were established early in Eng. for many of them (e.g. idea, arena) and many have crossed over since. It is now impossible to insist on purity
one way or the other without creating monstrosities.