The Roman Map of Britain Alaunus River Axe
Nauortium (R&C 241)
Alaunou potamou ekbolai Alauni fluvii ostia (Ptolemy II 3 3)
The Newerne ERN 302,
now known as The Lyd, seems a likely candidate.
Was the Newerne sufficiently prominent and so placed on the map that its name
was accidentally taken as that of the Severn?
Also in the vicinity is Nedern Brook which passed by Caerwent Venta Silurum. Loth suggested that Naurum was an error for Natrum and referred to the Nadder, which Ekwall considered doubtful.
The Neath is too distant.
Since Naurum is listed between the entries for the Tamar and the Avon, it should be somewhere in the vicinity of one or the other.
Some other rivers with compatible consonant clusters are n-v-r: Naver (Nabaros) and Nevern, n-r-n: Nairn and n-v: Noe (Navio).
If Sarua does not represent
the Severn, then perhaps Naurum deserves additional scrutiny.
Later in the river text of the Basel MS there is a lacuna that almost certainly represents a skipped line of text. Perhaps another lacuna common to all MSS between Tamaris and Naurum would explain the poor representation of south-western rivers.
If we take the N of Naurum
to represent an aggregated Al, then Alaunus fits well between
the Tamar and the Avon as the Axe. The reverse is seen in Ptolemy's Toliatis
for Tanatis, a dis-aggregation.
As it turns out this is one of the possibilities offered by PNRB p. 423, and one of the rare instances where PNRB relies on the cosmographer's consistency to make an argument.