Note by Samuel A. Stoner in St. John's Review [at page 61].
"This essay examines Virgil's account of the structure of Hell in Dante's Inferno. It has two goals. First, it aims to highlight the fact that Virgil's account of Hell is incomplete. Second, it seeks to explain why Virgil offers this incomplete account. It will pay special attention to the fact that Virgil does not account for the sixth circle of Hell or the heretics who are punished there."
See Dante, "On World Government" from De Monarchia in Gateway to the Great Books (10 Vol., 1963) volume 7, and The Divine Comedy in Great Books of the Western World (first edition, 52 Vol., 1952) volume 21, (second edition, 60 Vol., 1990) volume 19.