the best place to go for this is Greener's book altho there are other descriptive terms than the 4 he uses. i'm going off memory here as don't have the book any more but basically there are 4 aspects of a rib's shape: 2 longitudinal and 2 lateral. i believe greeners terms were straight, flat, curved and hollow. straight and curved are longitudinal, flat and hollow are lateral.
straight would give a straight line sighting plane from breech to muzzle. curved would follow the barrels (which in my mind is much more attractive as the rib doesn't show above the barrels when viewed from the side). flat would mean the rib were flat from one side to the other and hollow would be concave.
i think curved/hollow is what's usually referred to as swamped but i'm not certain of that.
roger
