(Μεγιστώ), is in some writers another form for Callisto, the mother of Arcas, who is also called Themisto. (Steph. Byz. s. v. Ἀργάς ; Eustath. ad Hom. p. 300; Hygin. Poet. Astr. 2.1.)
[L.S]A Dictionary of Greek and Roman biography and mythology
Smith, William
A Dictionary of Greek and Roman biography and mythology. William Smith, LLD, ed. 1890