This Nymphæum is represented on one of the coins of Alexander Severus, and belonged to the great Thermæ of the third century on the eastern side of Rome, which, having been long in progress, are called by the names of different emperors of that period. The ground had been originally the Exquiliæ, the great burial-ground of the time of the Republic, afterwards the garden of Mæcenas; it is full of aqueducts of different periods, and at different levels, and there are several reservoirs for them, some underground, others at a higher level. This one was formerly considered to have belonged to the Aqua Julia, but on taking the levels it appears that the only water in Rome that is high enough to reach it is the Anio Novus, the highest of the aqueducts, and the water has been brought from a reservoir belonging to that, near the Porta Tiburtina; having just been brought along the high bank on which the Wall of Rome stands, part of an arcade of an aqueduct leading to it remains, coming from that gate. Near the Porta Maggiore there are also remains of an aqueduct at a very high level, coming from the Anio Novus, but the construction of the arcade is of the third century. It is now part of the wall. This Nymphæum or reservoir at that high level has several branches leading from it, one of which goes in the direction of the other great reservoir on the Esquiline Hill, called the _Sette Sale_, and this, from the high level, probably supplied the Sette Sale and the Thermæ of Titus and Trajan. PLATE XIII. RIVER ALMO. DIVISION INTO TWO BRANCHES, NOW A LOCH OF THE MARRANA. [Illustration: XIII. RIVER ALMO, NOW THE MARRANA. DIVISION INTO TWO BRANCHES, NOW AT A LOCH OF THE MARRANA. PLAN, VIEW, AND SECTION.]