I am currently working through "Gradus ad Parnassum" in its english translation (Alfred Mann). In chapter One "Note against Note" (p. 27) I encounter "Principles for the forming of the melodies are discussed later". I have actually read through the First Part (Two Part Counterpoint) and there was hardly any discussion of cantus firmus rules. Should I be patient and read until the end or is it not covered in the book at all? I chose this book in the first place to have a complete and non-contradicting set of rules. In other sources (mostly online) there is much talk about the shape, climax etc., which I somehow miss here.
Mann cut off some sections from Fux's book. There's some discussion of this in Mann's translation, but I really don't remember where. Maybe next to the end of the book, after 4-voice 5th species.
But every counterpoint book now has a section reserved to it. You may look out for Jeppesen, Gauldin, Benjamin, Kennan, Thakar... the basic rules are most the same, with more or less different stylistic rules based on each author aesthetics or rethoric.
Also, this site is great as well: http://openmusictheory.com/cantusFirmus.html