The Via Valeria or Tiburtina leading to Tivoli, the ancient Tibur, now leaves Rome at the Porta S. Lorenzo. Traces of the polygonal pavement of the old road can be seen at intervals along the modern road to Tivoli, especially between the eighth and ninth milestones, and here and there elsewhere. In the Basilica of S. Lorenzo, a mile from the gate, are many ancient fragments of architecture. The Ponte Mammolo, by which the Anio is crossed at three and a half miles from Rome, is modern and there are scarcely any relics of the old bridge. Here and there on the road are the naked cores of tombs, but nothing of any interest offers itself to an archæologist until the Aquæ Albulæ are reached. Some few remains of an ancient building, which may have belonged to the Thermæ here, have been discovered. They are now built into the walls of a modern farmhouse. These ruins may have belonged to the Thermæ of Agrippa, which Augustus frequented. The ancient quarries of travertine mentioned by Strabo, whence the stone of the Coliseum came, lie on the right of the road beyond the Solfatara, and the modern quarries on the left. The road then crosses the Anio over an ancient bridge still called the Ponte Lucano, from Marcus Plautius Lucanus, a Tiburtine magistrate, whose memory is preserved in an inscription discovered upon the ancient fourteenth milestone on this road. The bridge was originally composed of three travertine arches, of which the one next to the left bank remains entire. The central arch has been restored with masonry of the sixth century, similar to that in the Ponte Nomentano and the Ponte Salario. The arch on the right bank was restored in the fifteenth century, and the whole bridge was repaired again about 1836. This bridge was broken down by Totila when he was encamped at Tibur, and Nibby thinks that he destroyed the middle arch, which was then restored by Narses. [Sidenote: Tomb of the Plautian family.] Just on the other side of the bridge is the tomb of the gens Plautia, well known from numerous paintings and photographs. It is very similar to that of Cæcilia Metella on the Appian Road, and to the Mausoleum of Hadrian, in its main features. A cylindrical tower of travertine, based on a square foundation, and capped with a cone, was the original design, but a mediæval tower built upon the top now disfigures it. Two inscriptions placed in a projecting front with Ionic pilasters record the names of M. Plautius Silvanus, consul with Augustus in the year B.C. 2, and his son Ti. Plautius Silvanus, prefect of the city in A.D. 73. A third inscription, which is now destroyed, commemorated a P. Plautius Pulcher. The longer inscription is given with notes in Wilmann’s Inscr. Lat. No. 1145. The person whose memory it preserves was the pontifex who officiated at the rebuilding of the Capitoline Temple in A.D. 70, as recorded by Tacitus. [Illustration: PLAN OF HADRIANEUM (_NEAR TIVOLI_)] [Sidenote: Hadrian’s Villa.]