Procopius's Brittia lies no farther than 200 stadia (25 miles) from the mainland, between Britannia (i.e. Brittany, at the time taken to include what is now Normandy, Flanders and part of Frisia, up to the mouths of Scheldt and Rhine) and Thule (Scandinavia), opposite the Rhine mouth, and three nations live in it, Angles, Frisians and Britons. Brittia thus corresponds to the island of Great Britain.
Procopius relates that
There have been suggestions as to at which point exactly these boats left the Gallic coast, Villemarqué placing it near Raz, Armorica, where there is a toponym baie des âmes / boé ann anavo "bay of souls".
Grimm reports that on the river Treguier in Bretagne, commune Plouguel, it is "said to be the custom to this day, to convey the dead to the churchyard in a boat, over a small arm of the sea called passage de l'enfer, instead of taking the shorter way by land".
Procopius's account is re-affirmed by Tzetzes (to Lycoph. 1204) in the 12th century; but long before that, Claudian at the beginning of the 5th (in Rufinum 1, 123-133) had heard of those Gallic shores as a trysting place of flitting ghosts. and not far from that region are Britain, the land of the Senones, and the Rhine. Grimm compares this account to the airy wagon of the Bretons, and to bardic traditions which make out that souls, to reach the underworld, must sail over the pool of dread and of dead bones, across the vale of death, into the sea on whose shore stands open the mouth of hell's abyss.