Mathias, I confess that I read Stewart's original message very quickly and without my glasses. Thus, I misread "resonantior" as "resonatur," in which case "forma . . . apta" would make sense as an ablative absolute, and focused primarily on the omission of "utravis" in his translation. Now that I am wearing my glasses, I can clearly see that "resonantior" is the comparative of the present participle and that both elements of the sentence lack a copula, which makes it clear that "forma" is nominative and not ablative absolute. In any event, it is nice to have another Latinist on the list, especially one who has also read Thomas' "De ente et essentia" in the original!
Best regards, Stephen To get on or off this list see list information at http://www.cs.dartmouth.edu/~wbc/lute-admin/index.html