I think it just used to function as a singular and so did cherise, lady-j. I don't know if the plurals were peases and cherises, or if they just stayed the same, like sheep.
I'm all for abandoning Greek and Latin plurals, bhg, I think they're just kept so old Etonians can swank. I think stadiums and syllabuses are fine though I know some people insist on stadia and syllabi, and I'd be happy with criterions too.
I think data and agenda have already made themselves English words. Data's some kind of collective noun as the concept of a singular datum barely exists. But agenda is a proper singular word with agendas as its plural.
jno - I agree with data. In my early days as a scientist I used to manipulate a sentence so that I could use "data" but in later days I gave up as it acquired its singular use became accepted.
sanmac: "TTT, the singular of dice is die. I think you meant something like sheep is both singular and plural. " - no I meant that many say dice when there is only one, when they should say die.