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
This Roman Coblentz displayed a pitiful spectacle in the high pretensions and paltry performances of the genteel world of Rome, their unseasonable reminiscences and still more unseasonable recriminations, their political perversities and financial embarrassments. It was a matter of comparatively slight moment that, while the old structure was falling to pieces, they were with the most painstaking gravity watching over every old ornamental scroll and every speck of rust in the constitution; after all it was simply ridiculous, when the genteel lords had scruples of conscience as to calling their deliberative assembly beyond the sacred soil of the city the senate, and cautiously gave it the title of the "three hundred";(23) or when they instituted tedious investigations in state law as to whether and how a curiate law could be legitimately enacted elsewhere than within the ring-wall of Rome.
23. As according to formal law the "legal deliberative assembly" undoubtedly, just like the "legal court," could only take place in the city itself or within the precincts, the assembly representing the senate in the African army called itself the "three hundred" (Bell. Afric. 88, 90; Appian, ii. 95), not because it consisted of 300 members, but because this was the ancient normal number of senators (i. 98). It is very likely that this assembly recruited its ranks by equites of repute; but, when Plutarch makes the three hundred to be Italian wholesale dealers (Cato Min. 59, 61), he has misunderstood his authority (Bell. Afr. 90). Of a similar kind must have been the arrangement as to the quasi-senate already in Thessalonica.
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Reference address : https://www.ellopos.net/elpenor/rome/5-10-brundisium-pharsalus-thapsus.asp?pg=56