The first known written use of the word Britain was an ancient Greek transliteration of the original P-Celtic term. It is believed to have appeared within a periplus by Pytheas, but no copies of this work survive. The earliest existing records of the word are quotations of the periplus by later authors, such as those within Strabo's "Geographica", Pliny's "Natural History" and Diodorus of Sicily's "Bibliotheca historica". According to Strabo, Pytheas referred to Britain as "Bretannikē", which is treated a feminine noun.