Wikipedia
Saint Mellonius was an early 4th-century Bishop of Rotomagus (now Rouen) in the Roman province of Secunda Provincia Lugdunensis (now Normandy in France). He is known only from a 17th-century 'Life' of little historical value, meaning the historicity of his existence is uncertain.
He is said to have been born near Cardiff in Wales, presumably at St Mellons, although the saint there is generally thought to be Saint Melaine, Bishop of Rennes. The two have, unfortunately, been hopelessly confused in many biographies. Mellonius' story tells how he travelled to Rome to pay the British tribute. He was there converted to Christianity by Pope Stephen I. He became the first Bishop of Rouen and died in AD 311. His feast day is 22 October. The church at Thiédeville is dedicated to him and he is said to have been baptised in a holy well at Héricourt. He may or may not be identical to the Bishop of Troyes of the same name who supposedly ruled in the 390s.
In the English translation of the 1956 edition of the Roman Martyrology, 'St Mellon' is listed under 22 October with the citation: At Rouen, St Mellon, Bishop, who was ordained by Pope St Stephen and sent thither to preach the Gospel.
In the 2004 edition of the Roman Martyrology, Mellonius is listed under the same date, 22 October, with the Latin name Mallóni. He is mentioned as follows: 'At Rothómagi (Rouen), bishop, who in that city announced the Christian faith and handed on the episcopate'.