Titus Livius (Livy), The History of Rome, Book 33
Cyrus Evans, Ed.

9. Quinctius, having received among the standards and ranks those who had been engaged with the enemy, gave the signal by sound of trumpet. [2] It is said, that such a shout was raised, as was seldom heard at the beginning of any battle; for it happened, that both armies shouted at once; not only the troops then engaged, but also the reserves, and those who were just then coming into the field. [3] The king, fighting from the higher ground, had the better on the right wing, by means chiefly of the advantage of situation. On the left, all was disorder and confusion; particularly when that division of the phalanx, which had marched in the rear, was coming up. [4] The centre stood intent on the fight as on a spectacle which in no way concerned them. The phalanx, just arrived, (a column rather than a line of battle, and fitter for a march than for a fight,) had scarcely mounted the top of the hill: [5] before these could form, Quinctius, though he saw his men in the left wing giving [6??] way, charged the enemy furiously, first driving on the elephants against them, for he judged that one part being routed would draw the rest after. The affair was no longer doubtful. [7] The Macedonians, repelled by the first shock of the elephants, instantly turned their backs; and the rest, as had been foreseen, followed them in their retreat. [8] Then, one of the military tribunes, forming his design in the instant, took with him twenty companies of men; left that part of the army which was evidently victorious; and making a small circuit, fell on the rear of the enemy's right wing. Any army whatever, thus charged from the rear, must have been thrown into confusion. But to that confusion which under such circumstances would be common to all armies, there was in this case an additional cause. [9] The phalanx of the Macedonians, being heavy, could not readily face about; nor would they have been suffered to do it by their adversaries in front, who, although they gave way to them a little before, on this new occasion pressed them vigorously. [10] Besides, they lay under another inconvenience in respect of the ground; for, by [p. 1449]pursuing the retreating enemy down the face of the hill, they had left the top to the party who came round on their rear. [11] Thus attacked on both sides, they were exposed for some time to great slaughter, and then betook themselves to flight, most of them throwing away their arms.