The Emperors Valentinian I and Valens divided the army among themselves. Many units were divided into two sub-units, sharing the name of the original unit and each distinguished by the name seniores, for the units of the senior Augustus Valentinian, and iuniores, for the units of the iunior Augustus Valens, and aggregated respectively to the Western Roman army and to the Eastern Roman army.Ammianus Marcellinus, xxvi.5.1-4.
