Since Palladio’s time, if not from a much earlier date, the Pons Fabricius has been known as the Ponte Quattro Capi, because its entrance from the left bank has a protective emblem, a quadrupled head of Janus, the guardian deity of gates, and a divinity with many other occupations, all very alert and troublesome. So we must add this pagan emblem to the other symbols of religious faith with which bridges have been sanctified. In 1680 the Pons Fabricius was repaired by Pope Innocent XI. There are two arches, each with a span of 25, 34 metres; and there used to be two other arches, only 3, 50 metres wide, pierced through the abutments, but they have disappeared among the houses on each bankside. The bridge in its greatest width measures a little more than 15 metres. It has a bold cornice ornamented with mutules, and its relief bay for spate water is flanked by pilasters. M. Degrand says of the Pons Fabricius: “C’est le premier pont dans lequel les têtes des voûtes ne forment pas des demi-circonférences: l’intrados est un arc de cercle de 25 m. de rayon et de 20 m. de flèche.” Here we find a starting-point for the lovely arch invented at Avignon by Saint Bénézet (p. 81). 3. The Pons Cestius, on the other side of the island, known to-day, and in Palladio’s time, as Ponte S. Bartolommeo. Yet its inscription, which is mentioned by Canina and by Sir William Smith, speaks of it as Pons Gratianus, and commemorates its repair by Valentinian, Valens, and Gratian. It has but one arch, nearly a metre less in span than those of the Pons Fabricius. These two bridges, according to Piranesi, were founded in a very remarkable manner, on reversed arches built under water. Gauthey gives two drawings of this construction, but he does not guarantee the truth of Piranesi’s details.