The following essay is an attempt to deal with an interesting branch of Roman military history which has not previously been made the subject of an independent treatise. In a study of this kind, which relies largely upon epigraphical evidence to which additions are constantly being made, it is equally necessary that the scattered material available should at intervals be collected and utilized, and that the unfortunate collector should realize that his conclusions will inevitably be revised in the future in the light of fresh evidence. I hope, accordingly, that I have made some use of all...