The first section of the Ravenna Cosmography to deal with Britain, covering 10546 to 1065, is obscure but nevertheless generally recognised as dealing with south-western England (Richmond & Crawford 1949, 13; Dillemann 1979, 65; Rivet & Smith 1979, 206). Why it should have been singled out by the Cosmographer is not at all clear. Rivet and Smith (1979, 197) see it as evidence for a special source covering this area in greater detail than the rest of Britain. This does not seem a necessary hypothesis. Indeed, the words that introduce the next section, ‘Again, next to the aforementioned civitas <Scadumnamorum>’ (iterum iuxta superscriptam ciuitatem <scadumnamorum>), strongly hint that the Cosmographer is looking at the same map as he used in this section.
We will see many instances of the Cosmographer duplicating names throughout his text, the most startling being *Moridunum (Sidford ?), which is repeated four times. However, they are not noticeably more common in this section than in those which follow. Had he employed a special and separate source for the south-west, it is difficult to see how he would have integrated the information he derived from it with that he derived from his main source without making many more such duplications. We would on this hypothesis also expect the following long section which covers the province or diocese of Britannia to contain a few names relating to the south-western peninsula which the Cosmographer had not noticed as duplications: we do not find them.
Arguments e silentio are never strong; more telling are the duplications within this section which cannot be the result of taking names from two different sources. For instance, the name *(Anti)uesteum appears twice, at 1061 and 1063, in both cases with virtually the same truncation. This truncation may well have occurred if the first three or four letters of the name were written ‘in the sea’ on the Cosmographer’s postulated map source (Rivet & Smith 1979, 198). The same error of reading is extremely unlikely to have occurred as a result of using two separate source documents.
There are thus no compelling reasons to believe that the Cosmographer was using a separate and fuller source for the south-west of Britain than for the remainder of the island. True enough, the density of names in the peninsula is high, but it is also high in Cumbria (1071 to 1076 and 10710 to 10711) and between the Roman walls (10730 to 10747). The contrast is not so much with a low density in the remainder of the province, but with specific areas, such as Wales and East Anglia, very poorly represented.
This does not solve the problem of why the Cosmographer should have seen (I)sca Dumnoniorum, Exeter, as a point at which to insert a break in his listing. The Peutinger Table may offer a clue: although Britain is severely truncated, with only East Anglia and Kent appearing on the surviving copy, Moridunum and Isca Dumnoniorum are also shown without any intervening south-coast places. It is possible that Isca Dumnoniorum was depicted as prominent in some way, perhaps isolated on a promontory or, as seems more likely, as the gateway to a peninsula (as suggested by Rivet & Smith 1979, 200). In this way the Cosmographer might have decided to break his text at a point which appeared dictated by the geography of the region. He does so further north, where his listing of the Antonine Wall forts occurs ‘where that same Britain is seen to be narrowest from sea to sea’ (ubi et ipsa britania plus angustissima de oceano in oceanum esse dinoscitur 10750 to 10751). Although this was not the primary reason for inserting a break at this latter point, the Cosmographer was clearly sensitive to the depicted shape of the island.
On the other hand, we should perhaps take into account the curious fact that the Civitas Dumnoniorum (basically the Cornish peninsula west of Exeter) appears to have been a part of Britain virtually unaffected by those changes to élite behaviour usually termed ‘romanisation’. Is it possible that much of it lay outside provincial or diocesan control and that some kind of border was depicted on the Cosmographer’s map source as separating the south-western peninsula from the rest of Britain? In that case, Isca Dumnoniorum may have been prominent as a point of contact between the wilds of the far south-west and the more ‘civilised’ Durotriges (or Durotrages, following RIB 1673: the form of the name is very uncertain according to Rivet & Smith 1979, 352) to the east.