Samuel Ball Platner, A Topographical Dictionary of Ancient Rome, London: Oxford University Press, 1929.

Because of Augustus, there were 14 regions in the city of Rome compared to its previous 4. This division into fourteen regions continued in force until the sevent century when an ecclesiastical division into seven regions was introduced and opened the way for the entirely different organization of the Middle Ages.

XIV, Trans Tiberim (Trastevere), all the city on the right bank of the Tiber, together with the insula Tiberina. The limits of this region cannot be determined, but it included much more than the territory within the Aurelian wall. It extended south as far as the temple of FORS FORTUNA (q.v.) and north far enough to include the Vatican district.

A Transtiberim [sic]: An indication of locality found on a sepulchral inscription of the empire (CIL vi. 9847)- the only instance known of Transtiberis as a noun.