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
Political Organization - Cantonal Constitution
The political development of the Celtic nation also presents very remarkable phenomena. The constitution of the state was based in this case, as everywhere, on the clan-canton, with its prince, its council of the elders, and its community of freemen capable of bearing arms; but the peculiarity in this case was that it never got beyond this cantonal constitution. Among the Greeks and Romans the canton was very early superseded by the ring-wall as the basis of political unity; where two cantons found themselves together within the same walls, they amalgamated into one commonwealth; where a body of burgesses assigned to a portion of their fellow- burgesses a new ring-wall, there regularly arose in this way a new state connected with the mother community only by ties of piety and, at most, of clientship.
Among the Celts on the other hand the "burgess-body" continued at all times to be the clan; prince and council presided over the canton and not over any town, and the general diet of the canton formed the authority of last resort in the state. The town had, as in the east, merely mercantile and strategic, not political importance; for which reason the Gallic townships, even when walled and very considerable such as Vienna and Genava, were in the view of the Greeks and Romans nothing but villages. In the time of Caesar the original clan-constitution still subsisted substantially unaltered among the insular Celts and in the northern cantons of the mainland; the general assembly held the supreme authority; the prince was in essential questions bound by its decrees; the common council was numerous--it numbered in certain clans six hundred members--but does not appear to have had more importance than the senate under the Roman kings. In the more stirring southern portion of the land, again, one or two generations before Caesar--the children of the last kings were still living in his time--there had occurred, at least among the larger clans, the Arverni, Haedui, Sequani, Helvetii, a revolution which set aside the royal dominion and gave the power into the hands of the nobility.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=26