There are two clues in Ptolemy's Geography (bk.II, ch.ii); near the the beginning of part 2, which reads:
"Description of the south side below which is the Oceanus Britannicus [English Channel]. After the Ocrium Promontorium [Lizard Point, Cornwall] is the mouth of the Cenio¹ river 14*003 51� (then) the mouth of the Tamarus² river 15*40 52� (then) the mouth of the Iscas³ river 17*40 52� ..."
and also the very last sentence of part 2:
"Next to these [the Durotriges], but more to the west, are the Dumnoni, whose towns are: Voliba 14*45 52� Uxella¹ 15*00 52� Tamara² 15*00 52� (and) Isca, where is located Legio II Augusta³ 17*30 52�."
The name Tamaris is mentioned in the Ravenna Cosmology (R&C#5) of the seventh century, where it is listed between the entries for NEMETOSTATIO (Nanstallon, Cornwall) and DVRNOVARIA (Dorchester, Dorset).
The river-name Tamar is possibly derived from Welsh?Gaelic, possibly meaning 'the dark one' or simply 'the river'. There are several other British rivers whose names have the same root-meaning; the Thames (Latin Tamesis) in London, the Team in County Durham, the Thame in Buckingham/Oxfordshire, and finally the Tame, of which there are three, in Warwickshire/Staffordshire, Yorkshire/Cheshire and North Yorkshire.
Ravenna Cosmography: Tamaris; Ptolemy: Tamara