ARNO (anc. Arnus)  , a river of Italy which rises from the Monte Falterona, about 25 M . E.N.E. of Florence, 4265 ft. above the sea . It first runs S.S.E. through a beautiful valley, the Casentino; near Arezzo it turns W., and at Montevarchi N.N.W.; 10 m. below it forces its way through the limestone rock at Incisa and 10 m. farther on, at Pontassieve, it is joined by the Sieve: Thence it runs westward to Florence and through the gorge of Golfolina onwards to Empoli and Pisa, receiving various tributaries in its course, and falls into the sea 71 M. west of Pisa, after a total course of 155 M . In prehistoric times the river ran straight on along the valley of the Chiana and joined the Tiber near Orvieto; and there was a great lake, the north end of which was at Incisa and the south at the lake of Chiusi . The distance from Pisa to the mouth in the time of Strabo was only 22 M . TheSerchio (anc . Auser), which joined the Arno at Pisa in ancient times, now flows into the sea independently . The Arno is navigable for barges as far as Florence; but it is liable to sudden floods, and brings down with it large quantities of earth and stones, so that it requires careful regulation . The most remarkable inundations were those of 1537 and 1740; in the former year the water rose to 8 ft. in the streets of Florence . The valley between Incisa and Arezzo contains accumulations of fossil bones of the deer, elephant, rhinoceros, mastodon, hippopotamus, bear, tiger, &c .