There are 10 Skull teeth total, 1 per world. You obtain them by completing chase levels where you must chase a treasure chest through a death trap and beat the uhh... tooth out of it at the end (on some levels this can be particularly satisfying after many trial and error attempts). Avoid hovering if you can as this will slow you down. Avoid unnecessary jumping and try to time every jump wisely. If you end up having to grab onto a ledge this will generally result in failure (on the later levels at least). Keep your momentum up and above all keep that button firmly pressed. If you are having troubles there are now full video playthroughs on all 10 skull teeth on YouTube.