For example, we have Venta Belgarum (Winchester), not Venta simply; Corinium Dobunorum (Cirencester), not Corinium simply.

The towns thus specially marked out are just those towns which are also declared by their actual remains to have been the chief country towns of Roman Britain.

This coincidence can hardly be an accident.

We may infer that the towns to which the Ravennas appends tribal names were the cantonal capitals of the districts of Roman Britain, and that a list of them, perhaps mutilated and imperfect, has been preserved by some chance in this late writer.

In other words, the larger part of Roman Britain was divided up into districts corresponding to the territories of the Celtic tribes; each had its capital, and presumably its magistrates and senate, as the above-mentioned inscription shows that the Silures had at Venta Silurum.
We may suppose, indeed, that the district magistrates--the county council, as it would now be called--were also the magistrates of the country town.