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
It was, no doubt, the duty of these to select the equites on military grounds and at their musters to insist that all horsemen incapacitated by age or otherwise, or at all unserviceable, should surrender their public horse; but the very nature of the institution implied that the equestrian horses should be given especially to men of means, and it was not at all easy to hinder the censors from looking to genteel birth more than to capacity, and from allowing men of standing who were once admitted, senators particularly, to retain their horse beyond the proper time.
Perhaps it was even fixed by law that the senator might retain it as long as he wished. Accordingly it became at least practically the rule for the senators to vote in the eighteen equestrian centuries, and the other places in these were assigned chiefly to the young men of the nobility. The military system, of course, suffered from this not so much through the unfitness for effective service of no small part of the legionary cavalry, as through the destruction of military equality to which the change gave rise, inasmuch as the young men of rank more and more withdrew from service in the infantry.
The closed aristocratic corps of the equites proper came to set the tone for the whole legionary cavalry, taken from the citizens who were of highest position by descent and wealth. This enables us in some degree to understand why the equites during the Sicilian war refused to obey the order of the consul Gaius Aurelius Cotta that they should work at the trenches with the legionaries (502), and why Cato, when commander-in-chief of the army in Spain, found himself under the necessity of addressing a severe reprimand to his cavalry.
But this conversion of the burgess-cavalry into a mounted guard of nobles redounded not more decidedly to the injury of the commonwealth than to the advantage of the nobility, which acquired in the eighteen equestrian centuries a suffrage not merely separate but giving the tone to the rest.
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Reference address : https://www.ellopos.net/elpenor/rome/3-11-government-governed.asp?pg=9