Eidumannia
Eidumannia in Britain, mentioned by Ptolemy (2.3.6) as a river between the prominence to the south of the Garrienus (Yare) and Orford Ness, and the Ἰάμισσα εἴσχυσις (sic in the current editions). Whatever may be the reading, this Ἰάμισσα means the Thames. Hence, the Stour, Orwell, Blackwater, or any of the Essex rivers may be the Eidumannia. The Black-water is the best; in which case the du==the Welsh du, Gaelic dubh==black.
From William Smith (ed.), Dictionary of Greek and Roman Geography. Text provided under a CC BY-SA license by Perseus Digital Library, http://www.perseus.tufts.edu, with funding from the National Endowment for the Humanities and Tufts University. Data accessed from https://github.com/PerseusDL/canonical-pdlrefwk/ on 17 October 2026.