With some nouns ending in -is, such as crisis, thesis, parenthesis, oasis, diagnosis are something of a special case. We balk at adding another 'es', although we don't seem to mind with "iris" => irises (not irides as in iridescent).
Again one might use 'brackets' or 'braces' instead of parentheses to get round the issue.

@WW Indeed.
It's not just Latin - there's the Greek stuff too - stigmata, schemata, phenomenon and criterion so forth. (i think I only ever teach the last two)
Teaching Latin in schools only serves to perpetuate these irregularities and delay the full assimilation of latin and greek words into English. Learning Spanish today would be of more use.

I cannot underwrite the idea that Latin is worth learning because of the "detective work" involved. There are plenty of other languages that offer just as much scope and are far more useful and relevant to everyday life.
I rate those youthful hours spent ploughing through the Aeneid as misguided and misspent, albeit at the time it was de rigeur. I think Winston and I are of the same mind on this.

"Merces/mercedis" is a case in point. In Vulgar latin it came to mean "favor" or "pity" and thence came to us via Old French as "mercy".

@ Brus the point about the latin is that if those of us with a smattering can't remember the plurals, what is the point of foisting them on the great unwashed speakers of barbarous mongrel tongues like Engish?

@Bru Copulis donatis sequitur:
1) merces is fem; plural mercedes (if you have one it is your rewards)
2) thus the verb is rightly 'est' and not "sunt"
3) the Vulgate uses "quoniam" not "quod" in this context and this is also quoted by the Pope Pius XII in EPISTULA ENCYCLICA ORIENTALES ECCLESIAS
Quod infallibilis est, non est corrigendum.
Veniam condonatumque spero. Nunc dimittis servum tuum.

@Brus cf Matt 5:12

@Brus
http://www.archives.nd.edu/cgi-bin/lookup.pl?stem=merces

cf Matt 5:12
http://www.latinvulgate.com/verse.aspx?t=1&b=1&c=5

reevaluationem, si placit

@Brus
nil lamentandum sed gaudete et exultate quoniam merces vestra copiosa est in caelis propter plurales latinis

Qualibus regulis? Qualis ludus? Quor sequor?