(ancient Arnus), the celebrated river of Florence, which rises in Monte Falterona, in the Apennines, and descends into the valley of Casentino, in Upper Tuscany. It passes the town of Bibbiena into the plain of Arezzo, where it receives the Chiana as a tributary, and enters the narrow valley of Larterna. Thence it issues through the rocky defile Dell' Inferno, pours into the lovely Val D'Arno, sweeps beneath the woods of Vallombrosa, and after receiving the Sieve, enters the plain of Florence, and proudly flows through the celebrated capital of Tuscany. Ten miles below Florence, it is confined in an artificial channel, formed by the ancient Etruscans, for the purpose of draining the plain. It receives several tributaries, and enters the plain of Pisa, which it traverses; and after a further course of eight miles, falls into the Tuscan sea. Its whole winding course is about 140 miles. At Florence it is 400 feet wide, but is fordable in summer. It is liable to sudden floods, and then is impetuous, carrying down with it immense quantities of Arno's earth and stones, by which its bed is elevated; in many places requiring frequent embankment. On the banks of the upper Arno, are vast accumulations of fossil bones of the elephant, rhinoceros, hippopotamus, and bear, especially between Arezzo and Florence.