Commentary Inf IV 140-141
For the absence of Musaeus from the familiar 'trinome' Orpheus-Musaeus-Linus, the first so-called 'theological poets' in various classical (e.g., Aristotle) and Christian (e.g., St. Augustine, St. Thomas) formulations see Holl.1984.2. The hypothesis of that discussion is that only in Virgil's fourth Eclogue would Dante have found Orpheus and Linus treated together and without Musaeus. Virgil's vaunt in his poem is that in his song he will outdo the efforts of Orpheus and Linus. But now we find him in Limbo with them, three classical failures in Dante's harsh, judgmental Christian view.

As for Seneca, as opposed to the widespread notion found in Toynbee (Seneca) and many others that Dante, like others in his time, believed that Seneca the moralist and Seneca the tragedian were two different persons, Mazzoni has argued (Mazz.1965.1, pp. 197-200), utilizing the work on the subject by Billanovich, that this confusion was only operative after Boccaccio's erroneous suppositions at mid-century (which eventually misled Petrarch into making the same mistake). For a fuller presentation of Toynbee's views, see his 'Dante and "Seneca morale"' (Toyn.1902.1), pp. 150-56.