Beyond the site of the Arval Chapel, the road passes by the relics of the papal palace of La Magliana, and then along a causeway six miles in length to Porto. The site of the ancient Portus Trajani on the right branch of the Tiber is now occupied by the town of Porto, mainly consisting of the cathedral, the Villa Pallavicini, and some farm buildings. The sea has here continuously receded for many centuries, and the river deposits of sand and marl have extended. Fiumicino at the present mouth of the river is two miles distant from Porto, and its site was entirely covered by the sea at the time when Claudius, and Trajan after him, constructed their new ports. The large marshy tract to the north of Porto marks the site of the port of Claudius. The hexagonal basin of Trajan lies between this marsh and the town of Porto. It is not at all clear when the right arm of the Tiber, or rather the canal which now serves for communication between the sea and the Tiber proper, assumed its present shape. Inundations and occasional repairs and alterations have changed its course, and the constant retreat of the sea must have lengthened it considerably. Nibby’s opinion is that, besides the large harbour, Claudius constructed an inner basin between the harbour and the old course of the river. Into this basin he cut a canal from the bend of the river near modern Ostia, and thus allowed the superfluous waters of the Tiber to escape through the harbour, and at the same time gained a supply of water for his docks. The inscription found in 1837, and now placed by the roadside near the Villa Pallavicini, alludes to this canal. The words of this inscription seem to show that the primary object of the canal was to supply the port with water, and that the advantage of preventing inundations at Rome was only secondary. Trajan probably enlarged and reconstructed the inner basin of Claudius, and surrounded it with the massive quays and warehouses, the ruins of which still remain. This inner basin is referred to by Juvenal. At the same time the canal was probably enlarged, and it is to this enlargement that Pliny alludes when he speaks of the canal formed by the most provident of all emperors.[154] [Sidenote: Ruins at Torre Paterno.]