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
Beyond doubt dealings of barter and traffic were carried on at these fairs long before the first Greek or Phoenician vessel entered the western sea. When bad harvests had occurred, different districts supplied each other at these fairs with grain; there, too, they exchanged cattle, slaves, metals, and whatever other articles were deemed needful or desirable in those primitive times. Oxen and sheep formed the oldest medium of exchange, ten sheep being reckoned equivalent to one ox.
The recognition of these objects as universal legal representatives of value or in other words as money, as well as the scale of proportion between the large and smaller cattle, may be traced back--as the recurrence of both especially among the Germans shows--not merely to the Graeco-Italian period, but beyond this even to the epoch of a purely pastoral economy.(16)
16. The comparative legal value of sheep and oxen, as is well known, is proved by the fact that, when the cattle-fines were converted into money-fines, the sheep was rated at ten, and the ox at a hundred asses (Festus, v. -peculatus-, p. 237, comp. pp. 34, 144; Gell. xi. i; Plutarch, Poplicola, ii). By a similar adjustment the Icelandic law makes twelve rams equivalent to a cow; only in this as in other instances the Germanic law has substituted the duodecimal for the older decimal system.
It is well known that the term denoting cattle was transferred to denote money both among the Latins (-pecunia-) and among the Germans (English fee).
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Reference address : https://www.ellopos.net/elpenor/rome/1-13-agriculture-trade-commerce.asp?pg=21