Note 2810 — BOOK IV. AN ACCOUNT OF COUNTRIES, NATIONS, SEAS, TOWNS, HAVENS, MOUNTAINS, RIVERS, DISTANCES, AND PEOPLES WHO NOW EXIST OR FORMERLY EXISTED.

This, according to Sillig, is the real meaning of a desertis here, the distance being measured from the Danube, and not between the Vistula and the wilds of Sarmatia. The reading “four thousand” is probably corrupt, but it seems more likely than that of 404 miles, adopted by Littré, in his French translation.