Who is “Megan the Astronomer”? On a cryptic reference in the 1536 Rabelais almanac

In the “Judgment of Bisexte” from his 1536 almanac, Rabelais gives an obscure reference: “Ptoleme in Isagoge ad megan astronomon.” Michael A. Screech, in his English translation, has rendered the passage as follows: “Ptolemy in his Isagoge for Megan the Astronomer.” It is worth looking at the bibliographies that exist, including lost or alleged works of Ptolemy, to find no mention of an introduction by this author for a certain Megan the Astronomer.