One of the features of the Natural History is the catalogue of topics and sources that comprises Book 1, a rare component of an ancient text. It provides the subjects of each book, followed by a list of the number of items within it. For Book 2 this is limited to “facts, investigations, and observations,” but for books 3–6 there is the number of “towns and peoples, famous rivers, famous mountains, islands, towns and peoples that have perished,” and then the summary, “facts, investigations, and observations.” The actual numbers for Books 3 and 4 are missing. Only the number of islands (118) survives for Book 5, and Book 6 has the complete sequence of numbers.