The Ems was known to several ancient authors: Pliny the Elder in Natural History (4.14), Tacitus in the Annals (Book 1), Pomponius Mela (3.3), Strabo and Ptolemy, Geography (2.10). Ptolemy's name for it was the Amisios potamos, and in Latin Amisius fluvius. The others used the same, or Amisia, or Amasia or Amasios. The identification is certain, as it always is listed between the Rhine and the Weser, and was the only river leading to the Teutoburg Forest.