The first two generations had a ring of buttons around the wheel rather than a touchwheel; menu at the top, play/pause at the bottom and skip backwards and forw

The first two generations had a ring of buttons around the wheel rather than a touchwheel; menu at the top, play/pause at the bottom and skip backwards and forwards on the left and right. The third gen moved these buttons to between the wheel and the screen. Fourth was clicky wheel, I believe.