Please note that Mommsen uses the AUC chronology (Ab Urbe Condita), i.e. from the founding of the City of Rome. You can use this reference table to have the B.C. dates
From: The History of Rome, by Theodor Mommsen
Translated with the sanction of the author by William Purdie Dickson
The Nobility in Possession of the Equestrian Centuries
The institution of the equites was developed into a second, less important but yet far from unimportant, organ of the nobility. As the new hereditary nobility had not the power to usurp sole possession of the comitia, it necessarily became in the highest degree desirable that it should obtain at least a separate position within the body representing the community. In the assembly of the tribes there was no method of managing this; but the equestrian centuries under the Servian organization seemed as it were created for the very purpose. The 1800 horses which the community furnished(12) were constitutionally disposed of likewise by the censors.
12. The current hypothesis, according to which the six centuries of the nobility alone amounted to 1200, and the whole equestrian force accordingly to 3600 horse, is not tenable. The method of determining the number of the equites by the number of duplications specified by the annalists is mistaken: in fact, each of these statements has originated and is to be explained by itself. But there is no evidence either for the first number, which is only found in the passage of Cicero, De Rep. ii. 20, acknowledged as miswritten even by the champions of this view, or for the second, which does not appear at all in ancient authors.
In favour, on the other hand, of the hypothesis set forth in the text, we have, first of all, the number as indicated not by authorities, but by the institutions themselves; for it is certain that the century numbered 100 men, and there were originally three (Cf. I. V. Burdens of the Burgesses), then six (I. Vi. Amalgamation of the Palatine and Quirinal Cities), and lastly after the Servian reform eighteen (Cf. I. VI. The Five Classes), equestrian centuries. The deviations of the authorities from this view are only apparent. The old self-consistent tradition, which Becker has developed (ii. i, 243), reckons not the eighteen patricio-plebeian, but the six patrician, centuries at 1800 men; and this has been manifestly followed by Livy, i. 36 (according to the reading which alone has manuscript authority, and which ought not to be corrected from Livy's particular estimates), and by Cicero l. c. (according to the only reading grammatically admissible, MDCCC.; see Becker, ii. i, 244).
But Cicero at the same time indicates very plainly, that in that statement he intended to describe the then existing amount of the Roman equites in general. The number of the whole body has therefore been transferred to the most prominent portion of it by a prolepsis, such as is common in the case of the old annalists not too much given to reflection: just in the same way 300 equites instead of 100 are assigned to the parent-community, including, by anticipation, the contingents of the Tities and the Luceres (Becker, ii. i, 238). Lastly, the proposition of Cato (p. 66, Jordan), to raise the number of the horses of the equites to 2200, is as distinct a confirmation of the view proposed above, as it is a distinct refutation of the opposite view.
The closed number of the equites probably continued to subsist down to Sulla's time, when with the -de facto- abeyance of the censorship the basis of it fell away, and to all appearance in place of the censorial bestowal of the equestrian horse came its acquisition by hereditary right; thenceforth the senator's son was by birth an -eques-. Alongside, however, of this closed equestrian body, the -equites equo publico-, stood from an early period of the republic the burgesses bound to render mounted service on their own horses, who are nothing but the highest class of the census; they do not vote in the equestrian centuries, but are regarded otherwise as equites, and lay claim likewise to the honorary privileges of the equestrian order.
In the arrangement of Augustus the senatorial houses retained the hereditary equestrian right; but by its side the censorial bestowal of the equestrian horse is renewed as a prerogative of the emperor and without restriction to a definite time, and thereby the designation of equites for the first class of the census as such falls into abeyance.
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Reference address : https://www.ellopos.net/elpenor/rome/3-11-government-governed.asp?pg=8