Cassius Dio, epit. LXXIX.4.1-5:


A.D. 217: It seems that a seer in Africa had declared, in such a manner that it became noised abroad, that both Macrinus the prefect, and his son, Diadumenianus, were destined to hold the imperial power; [2] and later this seer, upon being sent to Rome, had revealed this prophecy to Flavius Maternianus, who at the time commanded the soldiers in the city, and this man had at once written a letter to Antoninus [Caracalla]. But it happened that this letter was diverted to Antioch to the emperor's mother Julia, [3] since she had been instructed to sort everything that arrived and thus prevent a mass of unimportant letters from being sent to him while he was in the enemy's country; whereas another letter, written by Ulpius Julianus, who was then in charge of the census, went by other couriers direct to Macrinus, informing him of the state of affairs. Thus the message to the emperor was delayed, [4] while the dispatch to Macrinus was read by him in good season. And so Macrinus, fearing he should be put to death by Antoninus on this account, especially as a certain Egyptian, Serapio, had told the emperor to his face a few days earlier that he would be short-lived and that Macrinus would succeed him, delayed no longer. [5] Serapio had at first been thrown to a lion for this, but when, as the result of his merely holding out his hand, as is reported, the animal did not touch him, he was slain. (Loeb trans.)