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
Relations of the Social Question to the Question between Orders
The distinction between rich and poor, which arose out of these relations, by no means coincided with that between the clans and the plebeians. If far the greater part of the patricians were wealthy landholders, opulent and considerable families were, of course, not wanting among the plebeians; and as the senate, which even then perhaps consisted in greater part of plebeians, had assumed the superintendence of the finances to the exclusion even of the patrician magistrates, it was natural that all those economic advantages, for which the political privileges of the nobility were abused, should go to the benefit of the wealthy collectively; and the pressure fell the more heavily upon the commons, since those who were the ablest and the most capable of resistance were by their admission to the senate transferred from the class of the oppressed to the ranks of the oppressors.
But this state of things prevented the political position of the aristocracy from being permanently tenable. Had it possessed the self-control to govern justly and to protect the middle class--as individual consuls from its ranks endeavoured, but from the reduced position of the magistracy were unable effectually, to do--it might have long maintained itself in sole possession of the offices of state. Had it been willing to admit the wealthy and respectable plebeians to full equality of rights--possibly by connecting the acquisition of the patriciate with admission into the senate--both might long have governed and speculated with impunity. But neither of these courses was adopted; the narrowness of mind and short- sightedness, which are the proper and inalienable privileges of all genuine patricianism, were true to their character also in Rome, and rent the powerful commonwealth asunder in useless, aimless, and inglorious strife.
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Reference address : https://www.ellopos.net/elpenor/rome/2-02-tribunate-plebs-decemvirate.asp?pg=9