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
Agrarian Law of Spurius Cassius
More sagacious in plan than all these party steps was the attempt of Spurius Cassius to break down the financial omnipotence of the rich, and so to put a stop to the true source of the evil. He was a patrician, and none in his order surpassed him in rank and renown. After two triumphs, in his third consulate (268), he submitted to the burgesses a proposal to have the public domain measured and to lease part of it for the benefit of the public treasury, while a further portion was to be distributed among the necessitous.
In other words, he attempted to wrest the control of the public lands from the senate, and, with the support of the burgesses, to put an end to the selfish system of occupation. He probably imagined that his personal distinction, and the equity and wisdom of the measure, might carry it even amidst that stormy sea of passion and of weakness. But he was mistaken.
The nobles rose as one man; the rich plebeians took part with them; the commons were displeased because Spurius Cassius desired, in accordance with federal rights and equity, to give to the Latin confederates their share in the assignation. Cassius had to die. There is some truth in the charge that he had usurped regal power, for he had indeed endeavoured like the kings to protect the free commons against his own order. His law was buried along with him; but its spectre thenceforward incessantly haunted the eyes of the rich, and again and again it rose from the tomb against them, until amidst the conflicts to which it led the commonwealth perished.
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Reference address : https://www.ellopos.net/elpenor/rome/2-02-tribunate-plebs-decemvirate.asp?pg=28