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 Five Classes
Every freeholder from the eighteenth to the sixtieth year of his age, including children in the household of freeholder fathers, without distinction of birth, was under obligation of service, so that even the manumitted slave had to serve, if in an exceptional case he had come into possession of landed property. The Latins also possessing land--others from without were not allowed to acquire Roman soil--were called in to service, so far as they had, as was beyond doubt the case with most of them, taken up their abode on Roman territory.
The body of men liable to serve was distributed, according to the size of their portions of land, into those bound to full service or the possessors of a full hide,(6) who were obliged to appear in complete armour and in so far formed pre-eminently the war army (-classis-), and the four following ranks of smaller landholders--the possessors respectively of three fourths, of a half, of a quarter, or of an eighth of a whole farm--from whom was required fulfilment of service, but not equipment in complete armour, and they thus had a position below the full rate (-infra classem-).
6. -Hufe-, hide, as much as can be properly tilled with one plough, called in Scotland a plough-gate.
As the land happened to be at that time apportioned, almost the half of the farms were full hides, while each of the classes possessing respectively three-fourths, the half, and the quarter of a hide, amounted to scarcely an eighth of the freeholders, and those again holding an eighth of a hide amounted to fully an eighth. It was accordingly laid down as a rule that in the case of the infantry the levy should be in the proportion of eighty holders of a full hide, twenty from each of the three next ranks, and twenty-eight from the last.
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Reference address : http://www.ellopos.net/elpenor/rome/1-06-burgesses-reformed-constitution.asp?pg=13