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 consequences followed as a matter of course. The first and most essential condition of all aristocratic government is, that the plenary power of the state be vested not in an individual but in a corporation. Now a preponderantly aristocratic corporation, the senate, had appropriated to itself the government, and at the same time the executive power not only remained in the hands of the nobility, but was also entirely subject to the governing corporation.
It is true that a considerable number of men not belonging to the nobility sat in the senate; but as they were incapable of holding magistracies or even of taking part in the debates, and thus were excluded from all practical share in the government, they necessarily played a subordinate part in the senate, and were moreover kept in pecuniary dependence on the corporation through the economically important privilege of using the public pasture.
The gradually recognized right of the patrician consuls to revise and modify the senatorial list at least every fourth year, ineffective as presumably it was over against the nobility, might very well be employed in their interest, and an obnoxious plebeian might by means of it be kept out of the senate or even be removed from its ranks.
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Reference address : https://www.ellopos.net/elpenor/rome/2-01-constitution-magistrate.asp?pg=36