ταξιάρχης: The Roman taxiarchos

[Nathan Ross]

Zonaras is the only source that says Silvanus was an infantry commander

I was just looking through Julianus's Oration I  to Constantius - he is describing the same event here, but suggests that Silvanus was commanding cavalry: "Even before the battle at Myrsa, a division of cavalry (τάξις μὲν ἱππέων I think) came over to your side, and when you had conquered Italy bodies of infantry and distinguished legions did the same."