Caius Marius (1.1-1.2)

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.