ARNO (the ancient ARNUS), a celebrated river of Italy. It rises in Monte Falterona, in the Apennines, descends into the valley of Casentino, in Upper Tuscany, passes the town of Bibbiena into the plain of Arezzo, where it receives the Chiana as a tributary, and enters the narrow valley of Laterina. 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 flows through that city. 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 by an artificial embouchure excavated in 1603. 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 earth and stones, by which its bed is elevated ; in many places it requires frequent embankment. The most remarkable inundations are those of 1537 and 1740, the water on the former occasion rising 8 feet in the city of Florence. 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.