"Thames, Sir"

The Thames through Oxford is often given the name the River Isis, although historically, and especially in Victorian times, gazetteers and cartographers insisted that the entire river was correctly named the River Isis from its source until Dorchester-on-Thames. Only at this point, where the river meets the River Thame and becomes the "Thame-isis" (subsequently abbreviated to Thames) should it be so-called; Ordnance Survey maps still label the Thames as "River Thames or Isis" until Dorchester. However since the early 20th century, this distinction has been lost in common usage outside Oxford, and some historians suggest the name Isis—although possibly named after the Egyptian goddess of that name—is nothing more than a contraction of Tamesis, the Latin (or pre-Roman Celtic) name for the Thames.(Wikipedia)