The commanding officer of an ala quingenaria or miliaria, or of a cohors quingenaria bore the title of praefectus. Cohortes miliariae and the cohortes civium Romanorum, which occupied an exceptional position,[75] were commanded by tribuni. Early inscriptions also mention a subpraefectus alae and a subpraefectus cohortis, but these posts seem later to have been abandoned.[76] In later times in case of the absence of the praefectus, his place seems to have been filled by an officer placed temporarily in charge with the title of praepositus or curator. Questions concerning the order of precedence among the praefecti and tribuni, and their place in the military hierarchy generally, are so closely connected with the method of selection and appointment of these officers at different periods, that they are best left for future discussion.[77] It is only important here to note that they usually entered the service with this rank, and that it is very rare to find the regular commander either of an ala or cohort drawn from among the lower officers.
The remaining ‘commissioned officers’, as we should call them, are represented by the troop and company commanders, the decurions who commanded the turmae of the ala, and the centurions and decurions of the cohorts. The senior officer in each class was styled decurio princeps or centurio princeps,[78] but apart from this we cannot trace any regular order of precedence with fixed titles such as is found among the legionary centurions. As regards the respective position of infantry and cavalry officers, the decurio alae ranked highest. This is shown clearly, as von Domaszewski has pointed out, by the frequent employment of this officer as praepositus cohortis.[79] On the other hand, among the officers of the cohorts the centurions ranked above the decurions who commanded the mounted men, where such existed. In one inscription, which seems to have included all the officers of a cohors equitata, the centurions come first on the list, and in the Coptos inscription, so often cited, the officers of the 61 equites cohortales are not mentioned at all.[80] The difference in rank cannot, however, have been very great since all these officers could be promoted to the post of legionary centurion without any intervening step, although this distinction seems to have been conferred most freely upon the decurions of the alae. In these cases it was of course necessary for the auxiliary officer to have acquired the civitas either by serving his full time or by a special grant before his promotion.
Throughout the period these posts seem usually to have been filled by promotion from the lower ranks, although we also find instances of legionaries being given commissioned rank in the auxiliary regiments, and it is officers of this class who seem most frequently to have secured further promotion to the legionary centurionate.[81] Von Domaszewski wishes to consider that these transfers were especially characteristic of the early days of the imperial army, and that a deliberate attempt was then made to provide every auxiliary regiment with a staff of ex-legionaries. With this suggestion, however, it is difficult to agree; not only is the epigraphical evidence insufficient to prove such a wholesale use of imported officers, but the cases known to us are by no means confined to the first fifty years of the Empire. Further, as will be shown later, the arrangement does not harmonize with the general character of the early auxilia.
The holders of subordinate posts, who ranked below the centurion or decurion, may be divided, following the arrangement adopted by von Domaszewski, into two groups.[82] The members of the first group practically correspond to our non-commissioned officers, and are able to command small detachments or to take the place, if necessary, of the company officers. These alone, and the holders of certain higher administrative posts, to which the taktische Chargen gave access,[83] have a legitimate claim to the title of principales. The members of the second group did not, strictly speaking, rank above the privates, but they were granted freedom from certain routine duties in return for special services which they discharged, and were distinguished in consequence by the title of immunes.
It is of course often difficult to ascertain whether a particular post falls into the higher or lower group, and this is especially the case with the standard-bearers, who occupy a position of peculiar importance in the military system. In the ala each troop had its own flag carried by the signifer turmae, but there seems also to have been a regimental standard, the bearer of which was known as the vexillarius alae.[84] A few inscriptions also mention an imaginifer, but it is not clear whether this officer always or at all periods found a place on the staff.[85] In a cohort, on the other hand, each century seems to have had its signifer, and each turma of mounted men its vexillarius, but it does not appear that there was a regimental standard, any more than there existed at this date a standard for each cohort of a legion. This at least is implied by Tacitus in his description of the entry of the Vitellian army into Rome, when he mentions the alarum signa by the side of the legionum aquilae, but says nothing of the ensigns of the cohorts.[86] We must suppose, then, that the imaginifer cohortis, who is mentioned on inscriptions, was not regarded as the regimental standard-bearer any more than the imaginifer legionis.[87]
In consequence of this difference in organization the company and troop standard-bearers of the cohorts rank among the principales, while in the alae only the regimental standard-bearer is included in the higher group, and the signiferi turmae sink to the position of immunes.
Returning, then, to the ala we may place at the head of the principales the vexillarius, and next to him the imaginifer, when this officer existed. Other members of this class were the non-commissioned officers of every turma, the duplicarius and sesquiplicarius,[88] who derived their titles from the fact that they were paid twice and one and a half times the private’s pay respectively, an institution found in the Hellenistic military system from which it was probably borrowed.[89] Lastly we should perhaps add the optio, who commanded the escort of the praefectus (singulares).[90]
To the lower group, the immunes, belong the signifer, custos armorum, and curator attached to every turma,[91] the cornicularius,[92] actarius,[93] strator,[94] stator,[95] librarius,[96] and beneficiarius,[97] who form the clerical and administrative staff of the praefectus, and his escort, the singulares.[98] In determining the position of the holders of these posts among the immunes we are supported by the analogy of the Equites Singulares Imperatoris, a corps modelled upon and to a certain extent recruited from the auxiliary cavalry. The list of a turma of this regiment contained on a Roman inscription gives the following arrangement:[99]
nomina turmae
Iul(ius) Mascel(lus) dec(urio)
Nonius Severus dup(licarius)
Iul(ius) Victorinus sesq(uiplicarius)
Aur(elius) Mucatral
Aur(elius) Lucius
Ael(ius) Crescens sig(nifer)
Aur(elius) Victor arm(orum custos)
Aur(elius) Atero cur(ator)
Ael(ius) Victor bf (beneficiarius)
Cl(audius) Victorinus lib(rarius)
Iul(ius) Vindex bf (beneficiarius)
17 names of equites follow.