Wow, that is pretty strange to see. Well, I'm still thinkin' that originally Palestrina wouldn't have used only bass and treble clefs, that must have been a later addition of the editor. It looks to me like the notes should be played as read, with the treble clefs.
What's really strange to me are the O's between the two rows of clefs, and those brackets that're in the treble clefs for the tenor voices.
I'm really not sure, this is pretty odd. Would be curious to find out from someone who knows.