Many of the historiated initials illuminated by the Master of the Murano Gradual depict saints or subjects whose identity is not in doubt, because they have distinctive attributes; an example is St Stephen, who was stoned to death and is therefore shown with a stone and a bloody wound on his head.
But the subject of one illumination, a detail of which is shown above, is more perplexing than any other. It is one of a dozen illuminations by the Master of the Murano Gradual in the Wildenstein Collection at the Musée Marmottan, Paris. It is puzzling partly because it is so important: at about 540×365mm (c.21×14 inches) it is by far the largest illumination, and perhaps the only miniature (as opposed to a historiated initial) from the entire corpus of cuttings and single leaves attributed to the artist, usually thought to come from a dismembered volume of the multi-volume of Gradual of San Mattia, Murano. 
Before discussing what the miniature might represent, we should look at what it actually depicts:
|[Click to enlarge]|
The main scene shows thirteen men with haloes, presumably disciples or apostles (the one in the lower left corner is beardless, suggestive of St John the Evangelist). Two at the top are set off against the sky, separate from the other eleven. In the middle at the very top, between these men, there is a small white bird, presumably the Dove of the Holy Spirit, and a red feature that looks like fire or flames.
The left border has half-length monks, dressed alternately in black or white, presumably Benedictine and Camaldolese (as found in other related cuttings by the Master of the Murano Gradual), while the lower margin has three quatrefoils: the Virgin and Child flanked by two saints wearing mitres, presumably St Benedict and St Romuald (founders of the Benedictine and Camaldolese orders, respectively).
Between the miniature and its border there is a single word, "INCLITUS", below musical notation, that has never been satisfactorily explained, but which does at least prove that this leaf comes from some form of choirbook, such as an Antiphonary or Gradual.
Some parts of the miniature are clearly more important than others. The rabbits in the foreground and the trees in the background, for example, are unlikely to be significant except to indicate that the scene takes place outside. The dove and the fire in the sky, in contrast, cannot be insignificant, and it may be relevant that the dove is slightly to the left of centre, and the fire also seems to slant sideways, as if "pointing" towards one of the two men.
So what does the scene represent?
The earliest description of the miniature is in the 1838 Ottley auction catalogue, in which lot 192 is described as “The Descent of the Holy Spirit, a large sheet, splendidly gilt and coloured, in a border, containing in ten circular compartments, busts of Bishops and Monks, by the same hand”. The phrase "by the same hand" refers to the previous lot, described as “The Death of the Madonna, from Murano”; this is now at the Fitzwilliam Museum (discussed in this post).
So in 1838 the Wildenstein miniature was thought to represent "The Descent of the Holy Spirit", i.e. Pentecost. This is an understandable identification because it could represent the apostles, with tongues of fire coming from heaven, comparable to the description in Acts 2:1-3:
"And when the days of the Pentecost were accomplished, they were all together in one place: And suddenly there came a sound from heaven, as of a mighty wind coming, and it filled the whole house where they were sitting. And there appeared to them parted tongues as it were of fire, and it sat upon every one of them"
There are at least four serious objections to this identification, however: (i) the Virgin Mary is absent, but she is usually included in medieval representations of the scene; (ii) the tongues of fire are in the sky and are not "sat upon every one of them" as described in the Bible; (iii) the scene is set in a landscape, while the biblical account specifies that they were sitting in a house ("domum ubi erant sedentes" in the Vulgate), and almost all medieval depictions therefore show the scene taking place in an interior; and (iv) thirteen apostles are depicted, when there should be twelve.
The scene was still identified as Pentecost in 1926 when catalogued in the Edouard Kann collection, but in 1931 Paul Wescher, perhaps unhappy with the previous identification for some of the reasons listed above, referred to it instead as The Assumption of the Virgin .
In the most detailed published study to date, Mirella Levi D'Ancona did not accept either of these identifications, and described the scene as "The Mission of the Apostles". She writes,
"This scene is rarely depicted, and is more often fused with the scene of the Pentecost – from which it is distinguished mainly by the absence of the Virgin Mary and of the enclosure where the Apostles gathered when the Spirit of the Lord descended upon them". 
In the most recent article Bryan C. Keene and Stephanie Azzarello accept this identification, but they go further, by additionally identifying the music that appears below the miniature, the type of book from which the leaf comes, and the liturgical occasion on which it was sung :
"The Mission to the Apostles with a four-line musical stave and the opening line, Inclitus [pater rector ...] (from a Hymnal, Lauds for One Priest)".
I am very sceptical of all four aspects of this. Taking them in reverse order, it seems inconceivable to me that such a minor feast as "Lauds for One Priest" would warrant a full-page miniature. (Even Easter, usually the most important feast of the entire year, is not decorated as lavishly as this in the surviving companion volume in Berlin). There is no other surviving evidence for the existence of a Hymnal in this set of choirbooks, but if such a volume ever existed, with a nearly-full page miniature and full historiated border for one of the most minor texts, it must have been of almost unimaginable luxury, of which all other trace has been lost. As for the incipit, the text and music below the miniature consists of a single word, "Inclitus", so there is no way to know that it continued "pater rector", and overwhelming evidence to suggest that it did not .
Finally, there is the issue of the subject of the miniature itself. The iconographic subject called "The Mission to the Apostles" usually refers to an event described in Matthew 28:18-20 and Acts 1:8, when Jesus, after his Resurrection, and shortly before his Ascension, instructs his disciples to travel widely, preaching and baptizing: "And Jesus coming, spoke to them, saying: All power is given to me in heaven and in earth. Going therefore, teach ye all the nations ...", "be witnesses unto me in Jerusalem, and in all Judea, and Samaria, and to the ends of the earth." . In other words, if this really were The Mission to the Apostles, Jesus should be present, giving instructions, as for example in this version of the scene:
A second serious problem is that there were only eleven disciples at this point in the biblical narrative, because Judas had committed suicide in remorse for having betrayed Jesus . As mentioned above, one of the most distinctive feature of the Wildenstein miniature is that it depicts a group of thirteen men. Any satisfactory identification of the scene will have to explain this apparent discrepancy.
The Book of Acts begins by stating that Jesus spent forty days with his disciples after his Resurrection. He tells them that they should first wait in Jerusalem until baptized by the Holy Spirit (i.e. Pentecost), and then go out preaching and baptizing. He then ascends to heaven. They return to Jerualem, to the home of the remaining eleven disciples. Then Peter says that Judas must be replaced,
"And they appointed two, Joseph, called Barsabas, who was surnamed Justus, and Matthias. And praying, they said: Thou, Lord, who knowest the hearts of all men, show whether of these two thou hast chosen, To take the place of this ministry and apostleship, from which Judas hath by transgression fallen, that he might go to his own place. And they gave them lots, and the lot fell upon Matthias, and he was numbered with the eleven apostles." (Acts 1:23-26)
It seems to me that the this is what the miniature represents. The eleven surviving original disciples are gathered, together with Barsabas and Matthias, hence the group of thirteen. The Dove of the Holy Spirit and fire descends from heaven to indicate that Matthias, at the upper left, is the one chosen to replace Judas. The reason why this scene is represented at all, and on such a lavish scale, is explained by the fact that the manuscript was made for a monastery dedicated to St Matthias: San Mattia, Murano.
It is certainly not unusual for the dedication-saint of a monastery to be given very special treatment in its liturgical books, and a comparable situation is perhaps provided the large miniature of St George by the Murano Master's contemporary, Belbello da Pavia, cut from a choirbook made slightly later for another Venetian house, San Giorgio Maggiore:
Update, 20 Feb. 2022
Lucy Sandler has kindly contacted me to bring to my attention that there is a discussion in The Golden Legend about what is meant by the casting of lots to select Matthias in the biblical account. Here is the relevant passage from a modern translation, which suggests a very likely source for the idea that Matthias was selected by "a brilliant ray of light sent down from God upon Matthias", as depicted in the miniature (emphasis added):"They prayed, saying: “O Lord, you know the hearts of all! Show us which of these two brothers you have elected to take the place in this ministry and apostleship that Judas lost!” They cast lots and the choice fell on Matthias, who therefore was numbered with the eleven apostles. [ ... ] Regarding the kind of lots the above were, we have two opinions from the holy fathers. Jerome and Bede agree that they were the same kind of lots as those whose use we find very frequently in the Old Testament. On the other hand Dionysius, who was a disciple of Paul, considers it irreligious to hold this opinion and declares that as he sees it, this lot was nothing other than a brilliant ray of light sent down from God upon Matthias to show that he was to be received as an apostle."
 It is not certain whether the (much smaller) St Jerome at the Getty Museum is a miniature or a cropped historiated initial. It is something of an anomaly in the corpus of Murano Master cuttings; I therefore wonder if it might originally have been part of a choirbook of the church (completed in 1425) of San Girolamo, Venice, rather than the Gradual of San Mattia. Neither the Gettty Museum's online catalogue nor the recent article by Keene & Azzarello records the dimensions of the rulings, which might help settle the matter.
 Amédée Boinet, La collection de miniatures de M. Edouard Kann (Paris, 1926), p. 27 ("La Pentecôte"); Paul Wescher, Beschreibendes Verzeichnis der Miniaturen – Handschriften und Einzelblätter – des Kupferstichkabinetts der Staatlichen Museen Berlin (Leipzig, 1931), p. 110 (“Himmelfaht der Maria”).
 Mirella Levi D’Ancona, The Wildenstein Collection of Illuminations: The Lombard School, Storia della miniatura: Studi e documenti, 4 (Florence, 1970), p. 42.
 Stephanie Azzarello and Bryan C. Keene, ‘Splendors of the Serenissima in a Digital Age: The Master of the Murano Gradual Reconsidered’, Manuscript Studies: A Journal of the Schoenberg Institute for Manuscript Studies, 6.2 (2021), 223–67, at p. 260 no. 26.
 I am grateful to Innocent Smith who pointed out to me that Azzarello and Keene are citing a post-Vatican II (1960s) modification (found in the 1983 Liber Hymnarius) of a medieval hymn, "Inclitus pastor populique rector", and that the post-Vatican II version is actually "Inclitus rector pater", not "Inclitus pater rector". He also points out that there is a medieval Magnificat antiphon for St Tibertius which begins "Inclytus martyr Tiburtius ...", one for St Maurus beginning "Inclytus levita Christi Maurus ...", and some others. In sum, there is no reason to assume that the single word "Inclitus" belongs to a hymn; this leaf could therefore instead come from a Gradual and represent an antiphon, albeit one that is perhaps not recorded in sources such as the Cantus database.
 The Mission to the Apostles is also known as The Great Commission [Wikipedia].
 Two of the gospels are explicit about this number: "And the eleven disciples went into Galilee ..." (Matthew 28:16); "At length he appeared to the eleven as they were at table ..." (Mark 16:14).