66 notes on the first syllable of thronum[. . .] 70 notes on the last syllable of Jerusalem

The key to singing these properly is to be sure you follow the natural rhythm of the text.

OF. Domine Deus meus, in te speravi, end of v. 2

That rising minor seventh in Domine Deus meus sure isn't very common either. I'd be interested to know the provenance of that chant.

How about Precatus est Moyses and Reges Tharsis?

There is a 100 note melisma on the first syllable of "Caput" as I recall, in one of the antiphons sung during the washing of the feet in the Use of Sarum. The text recalls the moment where Peter refuses to have his feet washed by Jesus, Jesus responds he will "have not part of him" if he doesn't allow him to do so, and Peter then answers that he may wash not only his feet but his head (Caput) as well. The melisma on this word captures some of Peter's excitedness it seems

A little bit of info on this chant and some polyphony based on it:
https://en.wikipedia.org/wiki/Missa_Caput

The Sarum chant should be here... http://hmcwordpress.mcmaster.ca/renwick/missal/temporale/
but they have yet to get to Holy Week...

But I have a copy of the Sarum Graduale, Here is that last piece of the Mandatum,

So not a 100 note melisma.

N.B. If anyone wants more from the Sarum Graduale just ask.

I ran across this when--as a break from something else involving chant-- I entered a search for the longest melisma. The only one I have sung in a Mass is Iubilate Deo. As always, the problem with performance is psychological. Last time, when we first rehearsed it, we had all reached the same conclusion as to how to get through it, and that was memorize what I call the Coney Island Melisma because it--or rather, the whole phrase-- takes you up and down like a roller coaster. We sang it once and the conductor just said, 'You all certainly memorized that well." We were experienced, so there was little to rehearse. The psychological barrier comes during performance. I--and I was not alone--have had no trouble navigating the melisma. It was calming down and breathing properly for the next few phrases. If you watch The Voice, it is that thing that some coaches listen for: if another coach chooses a singer first, does the singer's performance fall apart, either from relaxing too early or from being too excited? Same thing: when you get through a breath-defying melisma, do you relax too early, or get too excited to keep going? Iubilate Deo is difficult that way in particular as the long melisma comes early on and there is much to do afterwords.

Kenneth

It is true that the melody of the melisma is not in any way the hardest part of the chant. That's implicit in what I wrote about--getting past the end of the melisma. Other parts of the chant--and some other chants with much shorter melismas--are trickier because of the less predictable patterns. I was referring to breathing, which for some of us is the big issue.

It's lucky that there was the qualifier in the original question that we were only to look at the gregorian repertory. I had a quick look through my pdf of the Antiphonale Ambrosianum, and only got up to the Third Sunday of Advent when I discovered these two chants. If I understand them correctly, both are melodies for the text "In populo" (that's not the incipit, that is the entire text).

The first has a melisma of 235 notes, and the second of a staggering 265 notes.

[EDIT: I guess it was inevitable I'd find a longer one. From the Nativity of the Lord, the second option on the next attachment for a melisma on the word "Deum" has a length of 336 notes]