It looks like a quilisma that has been filled in, but its seems to have a downward shape, as opposed to upward.

...note preceding...

Fr Columba and Gregorian Semiology would have it that the note after the quilisma is lengthened slightly, after being glided more or less rapidly up to, rather like a French baroque coulee. There has been virulent discussion of these conflicting interpretations on the Forum.

It is difficult to imagine that the followers of Fr Columba, Dom Cardine, and semiologists on the one hand, and followers of the so-called Solesmes Method on the other, will ever be reconciled. To whichever 'school of thought' one subscribes the most important matter is careful musicianship and bon gout - chant that has spirit and is alive.

...bringing out...

That is precisely the object of semiology, which is a text-driven art. All is subject to the conveyance of the text and its meaning and syntax. I have observed that followers of the so-called Solesmes Method (which according to Fr Columba no longer is - if it ever was - in force at Solesmes) lay great stress on how correctly (and sometimes 'preciously') to perform a salicus or whatever with never so much as taking notice of the text and its innate rhythm.

One might say that with the so-called Solesmes Method the music defines how the text is sung, whereas in semiology the text defines how the music is sung. Chant is all about the communication of the sacred verba, and this is the defining characteristic of semiology.

The OP's example is a salicus in the new graphics; the wavy note is the oriscus. More information here and the Latin original here. Anton Stingl uses this style of notation on the Gregor und Taube website. I believe Dom Cardine himself had called for a revision of the square notation. Why nobody has yet used a triplex notation but with modern stemless round notation is beyond me. If the square notation is inadequate, why are we so attached to it?

Since the quilisma interpretation just came up again, I'll use this opportunity to repost my question from yesterday on an older thread:

According to Gregorian Semiology, "the virga after the quilisma . . . is always followed by an intentional neumatic break" (p. 202). Why is the neumatic break here presumed to be intentional? How would one notate two or more ascending notes after a quilisma using the St. Gall neumes without a neumatic break? Is it possible?

Thanks @rarty. That is what I was looking for. There is still so much to learn about Chant. I'm looking forward to the full program to be released from Clear Creek later this year: http://lausinecclesia.com/

I've read and re-read the linked article by Peter Jeffery. I'm not totally clear on the application of the principle of lengthening the last note of a neume. Obviously, this is a matter of interpretation, but there must be some sort of consistency from one context to another.

III. A. The last note of a neume tends to regain the regular syllabic value [valorem syllabicum recuperandum], especially if it ascends again [after a downward movement].

The last part, "especially if it ascends again," isn't especially helpful. The illustration in section V shows a recovered syllabic beat at 6, the final, lowest note of a porrectus flexus. Does the tendency to lengthen the last note of a neume apply to three-note neumes such as the torculus and porrectus? To two-note neumes such as the clivis and podatus (pes rotundus)? I have listened to recordings from scholas that apply the most up-to-date semiological approach and don't hear evidence of the final note of each two- or three-note neume consistently being lengthened. Obviously, the last note of a neume with diminutive liquescence is not lengthened, as at note 26.

It is a very informative article and I wish to thank @rarty again for posting it in this thread.

I've spent the first part of my work day comparing recordings of the Easter introit, and here are my observations:

Claire: indistinguishable from "Old Solesmes," tonally and rhythmically, except breathing at quarter bars

Göschl: definite distinction between normal syllablic, diminished (i.e. melismatic), and augmented note values, but no noticeable tendency for the last note of a neume toward the so-called recovered syllabic value unless marked with an episema in the manuscripts; on the other hand, the practice of augmenting the value of the final syllable of a word when set to a single note (equivalent to a dotted note in the rhythmic editions) is clearly discernible

Joppich: somewhat similar to Göschl, but slower, and much more obvious repercussions on all repeated notes

Turco: interpretation almost exactly agrees with the principles in the LH preface

Gloria Dei Cantores: fastest tempo of all the recordings; no apparent tendency to lengthen the last note of a neume; at "tecum," the first (stressed) syllable is longer than the last

I don't recall anything in Gregorian Semiology suggesting either that the last note of a neume should return to the normal syllabic value, or that the last note of a word should be augmented if a single note. Rather, Cardine writes of the porrectus as three light notes, the torculus resupinus as four light notes, etc. I do remember reading in Agustoni-Göschl that the last note of a neume in the most important, but I don't have a copy of those volumes at my disposal. Is it possible that these are modern theories imposed on the chant, not entirely unlike Mocquereau's two- and three-note groupings? At least the latter do have some value for conducting/chironomy. After all, there has to be an ictus somewhere unless one beats every single note with a downward gesture.