What is "narnus"

Saint Narnus is venerated as the first bishop of Bergamo. Christian tradition holds that he was consecrated during the Apostolic Age in his office by St. Barnabas, although Narnus probably lived later than that. The oldest source that mentions Narnus dates from the 13th century; it was written by Branca da Gandino, a friar. He considers Narnus a bishop of Bergamo during the reign of Diocletian in the fourth century.