 Part of the rivers Danubius or Istros was also known as (together with the Black Sea) the "Okeanos" in ancient times, being called the "Okeanos Potamos" (Okeanos River).The lower Danube was also called the "Keras Okeanoio" (Gulf or Horn of Okeanos) in the "Argonautica" by Apollonius Rhodos (Argon.IV.