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 best-known incident in these conflicts of the orders is the history of Gnaeus Marcius, a brave aristocrat, who derived his surname from the storming of Corioli. Indignant at the refusal of the centuries to entrust to him the consulate in the year 263, he is reported to have proposed, according to one version, the suspension of the sales of corn from the state-stores, till the hungry people should give up the tribunate; according to another version, the direct abolition of the tribunate itself.
Impeached by the tribunes so that his life was in peril, it is said that he left the city, but only to return at the head of a Volscian army; that when he was on the point of conquering the city of his fathers for the public foe, the earnest appeal of his mother touched his conscience; and that thus he expiated his first treason by a second, and both by death. How much of this is true cannot be determined; but the story, over which the naive misrepresentations of the Roman annalists have shed a patriotic glory, affords a glimpse of the deep moral and political disgrace of these conflicts between the orders.
Of a similar stamp was the surprise of the Capitol by a band of political refugees, led by a Sabine chief, Appius Herdonius, in the year 294; they summoned the slaves to arms, and it was only after a violent conflict, and by the aid of the Tusculans who hastened to render help, that the Roman burgess-force overcame the Catilinarian band.
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Reference address : https://www.ellopos.net/elpenor/rome/2-02-tribunate-plebs-decemvirate.asp?pg=24