Canto XX of the Inferno raises some interesting questions about Dante's use of classical sources in The Divine Comedy, and his use of the character Virgil as well.
On a first reading, this canto is just one in a series of lists of sinners, both real and mythological. Virgil's monologue takes up almost the entire canto; he pauses periodically to allow Dante to give appropriately respectful replies. The sinners here are all diviners, and, as is typical of the Inferno, the punishment is suited to the crime--their heads are grotesquely turned around backwards, so that they cannot see anything in front of them. We are introduced to Amphiaraus, Tiresies, Arnus, Manto, Euryphylus, Michael Scott, and Guido Banatti, and told, breifly, the situations of each. In six of the seven cases the facts are either twisted horribly, or just plain wrong. Critics have for centuries found excuses for Dante's errors here, citing errors in the texts he was working from, and have not been terribly bothered by the fact that no such erroneous texts have been found.
The case of Euryphylus is especially interesting.
Then said he to me: "He who from the cheek
Thrusts out his beard upon his swarthy shoulders
Was, at the time when Greece was void of males,
So that there scarce remained one in the cradle,
An augur, and with Calchas gave the moment,
In Aulis, when to sever the first cable.
Eryphylus his name was, and so sings
My lofty Tragedy in some part or other;
That knowest thou well, who knowest the whole of it.
The only account of Euryphylus in the Aeneid is a lie. He is mentioned in an account told to the Trojans that the Greeks have left, thereby convincing them to accept the Trojan horse. It is bold enough of Dante to quote a liar, but then one must remember that DANTE is not quoting anything--Dante (the poet) makes Virgil (the character) give these histories, thereby contradicting himself.
The seeming irrelevant (and ridiculously detailed) account Virgil gives of the founding of Mantua in this same canto is even more telling. He claims that Mantua was named for an old virgin witch who had died on that spot before, and that the town bears no other connection to her. Then he says:
Therefore I caution thee, if e'er thou hearest
Originate my city otherwise,
No falsehood may the verity defraud."
And I: "My Master, thy discourses are
To me so certain, and so take my faith,
That unto me the rest would be spent coals.
Let's think: where might Dante have heard a different version of this story? How about the Aeneid? Virgil (the poet) claims that Mantua was founded by the son of Mantos, who then named it after her. Dante, who (as his Virgil says) "knowest the whole of it," knows very well what he is doing. His is not simply a whole-hearted admiration of Virgil and devotion to the classics in general--he contradicts them, and intends to supplant them.
A similar situation can be seen several cantos earlier, in his treatment of Brunetto Latini in Canto XV. Brunetto was a friend of Dante's, a mentor, and Dante (the traveller) is surprised to see him in Hell, and is sympathetic and courteous. Now, before we get overwhelmed by the mushiness of this reunion, perhaps we should ask: who's writing this? Who put him in Hell? Dante condemns Brunetto for sodomy; no other records have been found to this effect. Dante may have liked, admired, and respected Brunetto, but he was very deliberately ruining his reputation here.
So what are we to make of this? Dante, clearly, had an extraordinarily high opinion of himself, as his cruel treatment of Virgil and Brunetto Latini shows. However, critics for centuries have taken Dante's admiration for Virgil at face value, and severely underestimated the depth of craftsmanship involved here. So maybe Dante was right after all.