The Roman Map of Britain Nixa?

Mixa var. a lacuna in the Paris ms. (R&C 229) next

Mixa may be /Nixa, for Nixsa or Nessa

    The recorded forms are Inverness Invernis a.1300, Invirnisse ca.1310; river and loch Nesa, Nisa a.700, Nis a.1300.
Abernethy, near Perth, is Aburnethige, Apurnethige ca.970; near Grantown, Abirnethi 1461. 
    Root of nixie (niks-a) 'water-spirit'.
    For the River Ness, Nicolaisen has *Nessos or *Nessa < *Nestos or *Nesta < *Nedtos, *Nedta 'stream, river' <IE *ned- 'to wet, flood'.