Commentary Par XII 134-135

'Petrus Hispanus (Pedro Juliani), born at Lisbon, c. 1225, where he at first followed his father's profession of medicine; he studied at Paris, probably under Albertus Magnus; subsequently he was ordained and became (1273) archbishop of Braga; in 1274 he was created cardinal bishop of Tusculum (Frascati) by Gregory X; on Sept. 13, 1276, he was elected pope, under the title of John XXI, at Viterbo, in succession to Adrian V; he died May 20, 1277, after a reign of a little more than eight months, his death being caused by the fall of the ceiling of one of the rooms in his palace at Viterbo' (Ispano, Pietro).  His manual of logic, the Summulae logicales, in twelve books, had a large audience.

That Dante calls no attention whatsoever to the fact that Peter was a pope (if very briefly) has caught the attention of many commentators.  For the 'scorecard' of the perhaps twelve popes who, in Dante's opinion, were saved (and the probably larger number who were damned), see C.Inf.VII.46-48.  John XXI is the last saved pope mentioned in the poem.