The earliest written references that have survived relating to the islands were made by Pliny the Elder in his "Natural History", where he states that there are 30 ', and makes a separate reference to "Dumna", which Watson (1926) concludes is unequivocally the Outer Hebrides. Writing about 80 years later, in 140–150 AD, Ptolemy, drawing on the earlier naval expeditions of , also distinguished between the ', of which he writes there were only five (and thus possibly meaning the Inner Hebrides) and . ' is cognate with the Early Celtic "dumnos" and means the "deep-sea isle". Pliny probably took his information from Pytheas of Massilia who visited Britain sometime between 322 and 285 BC. It is possible that Ptolemy did as well, as Agricola's information about the west coast of Scotland was of poor quality. Breeze also suggests that might be Lewis and Harris, the largest island of the Outer Hebrides although he conflates this single island with the name "Long Island". Watson (1926) states that the meaning of Ptolemy's ' is unknown and that the root may be pre-Celtic. Murray (1966) claims that Ptolemy's ' was originally derived from the Old Norse ', meaning "isles on the edge of the sea". This idea is often repeated but no firm evidence of this derivation has emerged.