The collection, as it has reached us, consists of nearly a hundred pieces. Of these three-fourths are fables proper; being not so much translations from the Greek of Aesop as versions of the traditional stories, written and unwritten, which were the common inheritance of the Aryan peoples. Mixed up with these are a number of stories which are not strictly fables; five of them are about Aesop himself, and there are also stories told of Simonides, Socrates, and Menander. Two are from the history of his own time, one relating a grim jest of the Emperor Tiberius, and the other a domestic tragedy which had been for a while the talk of the town in the previous reign. There are also, besides the prologues and epilogues of the several books, a few pieces in which Phaedrus speaks in his own person,[10] defending himself against detractors with an acrid tone which recalls the Terentian prologues. The body of fables current in the Middle Ages is considered by the most recent investigators to descend from the collection of Phaedrus, though probably supplemented from the Greek collection independently formed by Babrius about the same period. Though Livy is the single great historian of the Augustan age, there was throughout this period a profuse production of memoirs and commentaries, as well as of regular histories. Augustus wrote thirteen books of memoirs of his own life down to the pacification of the Empire at the close of the Cantabrian war. These are lost; but the _Index Rerum a se Gestarum,_ a brief epitome of his career, which he composed as a sort of epitaph on himself, is extant. This document was engraved on plates of bronze affixed to the imperial mausoleum by the Tiber, and copies of it were inscribed on the various temples dedicated to him in many provincial cities after his death. It is one of these copies, engraved on the vestibule wall of the temple of Augustus and Rome at Ancyra in Galatia, which still exists with inconsiderable gaps. His two principal ministers, Maecenas and Agrippa, also composed memoirs. The most important work of the latter hardly, however, falls within the province of literature; it was a commentary on the great geographical survey of the Empire carried out under his supervision.