Titus Livius (Livy), The History of Rome, Book 33
Evan T. Sage, Ph.D. Professor of Latin and Head of the Department of Classics in the University of Pittsburgh, Ed.

4. Against the defeat sustained in the narrows at the Aous river he set the triple defeat inflicted at Atrax by the Macedonian phalanx upon the Romans. [2] Even there, when they had failed to hold the passes of Epirus which they commanded, the blame rested first on those who had not maintained careful vigilance, next, in the actual battle, on the light infantry and the mercenaries; [3] the Macedonian phalanx, on the other hand, had stood fast even then, and would always stand unconquered when regular battle was joined on level ground. [4] There were sixteen thousand in the phalanx, the flower of the whole kingdom. In addition, there were two thousand with light shields, whom they call peltasts, and an equal number (two thousand each) of Thracians and Illyrians —Tralles [5] is the name of the tribe —and auxiliary mercenaries from different nationalities to the number of about fifteen hundred [p. 287]and two thousand cavalry. With these forces the¹ king awaited the enemy. [6] The Romans had about the same number; it was only in cavalry strength that they were superior, because of the arrival of the Aetolians.