Of a third name for Caius Marius we are ignorant, as we are in the case of Quintus Sertorius the subduer of Spain, and of Lucius Mummius the captor of Corinth; for Mummius received the surname of Achaïcus from his great exploit, as Scipio received that of Africanus, and Metellus that of Macedonicus.
From this circumstance particularly Poseidonius thinks to confute those who hold that the third name is the Roman proper name, as, for instance, Camillus, Marcellus, or Cato; for if that were so, he says, then those with only two names would have had no proper name at all. But it escapes his notice that his own line of reasoning, if extended to women, robs them of their proper names; for no woman is given the first name, which Poseidonius thinks was the proper name among the Romans.
Moreover, of the other two names, one was common to the whole family, as in the case of the Pompeii, the Manlii, or the Cornelii (just as a Greek might speak of the Heracleidae or the Pelopidae), and the other was a cognomen or epithet, given with reference to their natures or their actions, or to their bodily appearances or defects, Macrinus, for example, or Torquatus, or Sulla (like the Greek Mnemon, Grypus, or Callinicus). 1
However, in these matters the irregularity of custom furnishes many topics for discussion.