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
A prose literature arose in Rome not much later than Roman poetry, but in a very different way. It experienced neither the artificial furtherance, by which the school and the stage prematurely forced the growth of Roman poetry, nor the artificial restraint, to which Roman comedy in particular was subjected by the stern and narrow-minded censorship of the stage. Nor was this form of literary activity placed from the outset under the ban of good society by the stigma which attached to the "ballad-singer." Accordingly the prose literature, while far less extensive and less active than the contemporary poetical authorship, had a far more natural growth.
While poetry was almost wholly in the hands of men of humble rank and not a single Roman of quality appears among the celebrated poets of this age, there is, on the contrary, among the prose writers of this period hardly a name that is not senatorial; and it is from the circles of the highest aristocracy, from men who had been consuls and censors--the Fabii, the Gracchi, the Scipios--that this literature throughout proceeds.
The conservative and national tendency, in the nature of the case, accorded better with this prose authorship than with poetry; but here too--and particularly in the most important branch of this literature, historical composition--the Hellenistic bent had a powerful, in fact too powerful, influence both on matter and form.
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Reference address : https://www.ellopos.net/elpenor/rome/3-14-literature-art.asp?pg=84