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
As his sons were not yet grown up, the reins of government were practically held by an illegitimate nephew of the king, the prince Jugurtha. Jugurtha was no unworthy grandson of Massinissa. He was a handsome man and a skilled and courageous rider and hunter; his countrymen held him in high honour as a clear and sagacious administrator, and he had displayed his military ability as leader of the Numidian contingent before Numantia under the eyes of Scipio.
His position in the kingdom, and the influence which he possessed with the Roman government by means of his numerous friends and war-comrades, made it appear to king Micipsa advisable to adopt him (634), and to arrange in his testament that his own two elder sons Adherbal and Hiempsal, and his adopted son Jugurtha along with them, should jointly inherit and govern the kingdom, just as he himself had done with his two brothers. For greater security this arrangement was placed under the guarantee of the Roman government.
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Reference address : https://www.ellopos.net/elpenor/rome/4-04-rule-restoration.asp?pg=28