Casius or Casium (Ancient Greek: Κασιον, Kasion) was a residential episcopal see in the Roman province of Augustamnica Prima in Lower Egypt, and is now a titular see of the Catholic Church.
[1] The article about it in the Catholic Encyclopedia of 1908 calls the see "Casium",[2] but the official yearbook of the Holy See gives "Casius" as the Latin form (and "Casio" in Italian).
[1] The city that gave its name to the see was not far from Pelusium and close to the sandhills known to Greek geographers as the Casium Mountain (Κασιον Ορος, Kasion Oros), today Ras Kouroun, El-Katieh, or El-Kas.
The town is mentioned in Georgius Cyprius, Hierocles's Synecdemos (727, 2), and Parthey's Notitia Prima, about 840, as a bishopric depending on Pelusium.
Saint Cyril of Alexandria sent him, together with Hermogenes, Bishop of Rhinocorua, to Rome, where both were present at the consecration of Pope Sixtus III.