VELLEIUS PATERCULUS, History of Rome
[II.XCIII] Ante triennium fere quam Egnatianum scelus erumperet, circa Murenae Caepionisque coniurationis tempus, abhinc annos L, M. Marcellus, sororis Augusti Octauiae filius, quem homines ita, si quid accidisset Caesari, successorem potentiae eius arbitrabantur futurum, ut tamen id per M. Agrippam securo ei posse contingere non existimarent, magnificentissimo munere aedilitatis edito, decessit, admodum iuuenis, sane, ut aiunt, ingenuarum uirtutum laetusque animi et ingenii fortunaeque in quam alebatur capax. Post cuius obitum, Agrippa, qui sub specie ministeriorum principalium profectus in Asiam, ut fama loquitur, ob tacitas cum Marcello offensiones, praesenti se subduxerat tempori, reuersus inde, filiam Caesaris Iuliam quam in matrimonio Marcellus habuerat duxit uxorem, feminam neque sibi neque rei publicae felicis uteri.
[2.93] Some three years before the plot of Egnatius was exposed, about the same time of the conspiracy of Murena and Caepio, fifty years from the present date, Marcus Marcellus died, the son of Octavia, sister of Augustus, after giving a magnificent spectacle to commemorate his aedileship and while still quite a youth. People thought that, if any thing should happen to Caesar, Marcellus would be his successor in power, at the same time believing, however, that this would not fall his lot without the opposition of Marcus Agrippa. He was, we are told, a young man of noble qualities, cheerful in mind and disposition, and equal to the station for which he was being reared. After his death, Agrippa, who had set out for Asia on the pretext of commissions from the emperor. but who, according to current gossip, had withdrawn for the time being, on account of his secret animosity for Marcellus, now returned from Asia and married Julia, the daughter of Caesar, who had been the wife of Marcellus, a woman whose many children were to be blessings neither to herself or to the state.