Pliny, Natural History 1, p. ix. A possible source for the passage under discussion is Marcus Agrippa, who wrote before 12 B.C.E.; he is the first authority whom Pliny lists for the information in Book 5 (the lengthy catalogue of sources constitutes the first book of Pliny’s composition). See the comments of J.J. Tierney, “The Map of Agrippa,” Proceedings of the Royal Irish Academy, vol. 63, section C, no. 4 (Dublin: Hodges, Figgis, 1963), p. 155. I thank my colleague Stephen Goranson for this reference.