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
That they did so cannot of course be strictly proved; but as the use of masks for the face in Latium in the case of the national drama was of immemorial antiquity, while the Greek drama in Rome did not adopt them for a century after its first establishment, as, moreover, those Atellane masks were of decidedly Italian origin, and as, in fine, the origination as well as the execution of improvised pieces cannot well be conceived apart from fixed masks assigning once for all to the player his proper position throughout the piece, we must associate fixed masks with the rudiments of the Roman drama, or rather regard them as constituting those rudiments themselves.
Earliest Greek Influences
If our information respecting the earliest indigenous culture and art of Latium is so scanty, it may easily be conceived that our knowledge will be still scantier regarding the earliest impulses imparted in this respect to the Romans from without. In a certain sense we may include under this head their becoming acquainted with foreign languages, particularly the Greek.
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Reference address : https://www.ellopos.net/elpenor/rome/1-15-art.asp?pg=14