John XXI (pope)
John XXI, d. 1277, pope (1276–77), a Portuguese named Pedro Giuliano; successor of Adrian V. Known generally as Peter of Spain (Petrus Hispanus), he is the only Portuguese pope. Peter's reputation as a scholastic philosopher was widespread, and he was the reputed author of an extensively used book on logic. Nine months after his election the ceiling of his library at Viterbo fell and killed him. Due to confusion in the medieval listing of popes named John, he called himself John XXI instead of John XX; this numbering is usually maintained. He was succeeded by Nicholas III.
"John XXI (pope)." The Columbia Encyclopedia, 6th ed.. . Encyclopedia.com. (September 22, 2018). http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/john-xxi-pope
"John XXI (pope)." The Columbia Encyclopedia, 6th ed.. . Retrieved September 22, 2018 from Encyclopedia.com: http://www.encyclopedia.com/reference/encyclopedias-almanacs-transcripts-and-maps/john-xxi-pope