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 Nobility in Possession of the Senate
The dependence -de jure- of the Roman senate of the republic, more especially of the larger patricio-plebeian senate, on the magistracy had rapidly become lax, and had in fact been converted into independence. The subordination of the public magistracies to the state-council, introduced by the revolution of 244;(9) the transference of the right of summoning men to the senate from the consul to the censor;(10) lastly, and above all, the legal recognition of the right of those who had been curule magistrates to a seat and vote in the senate,(11) had converted the senate from a council summoned by the magistrates and in many respects dependent on them into a governing corporation virtually independent, and in a certain sense filling up its own ranks; for the two modes by which its members obtained admission--election to a curule office and summoning by the censor--were both virtually in the power of the governing board itself.
9. Cf. II. I. Government of the Patriciate
10. Cf. II. III. Censorship
11. Cf. II. III. The Senate
The burgesses, no doubt, at this epoch were still too independent to allow the entire exclusion of non-nobles from the senate, and the nobility were perhaps still too judicious even to wish for this; but, owing to the strictly aristocratic gradations in the senate itself--in which those who had been curule magistrates were sharply distinguished, according to their respective classes of -consulares-, -praetorii-, and -aedilicii-, from the senators who had not entered the senate through a curule office and were therefore excluded from debate--the non-nobles, although they probably sat in considerable numbers in the senate, were reduced to an insignificant and comparatively uninfluential position in it, and the senate became substantially a mainstay of the nobility.
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Reference address : http://www.ellopos.net/elpenor/rome/3-11-government-governed.asp?pg=7