in alligators, the upper jaw is wider than the lower jaw and completely overlaps it. Therefore, the teeth in the lower jaw are almost completely hidden when the mouth closes, fitting neatly into small depressions or sockets in the upper jaw. This is particularly apparent with the large fourth tooth in the lower jaw. In crocodiles, the upper jaw and lower jaw are approximately the same widths and so teeth in the lower jaw fit along the margin of the upper jaw when the mouth is closed. Therefore, the upper teeth interlock (and “interdigitate”) with the lower teeth when the mouth shuts. As the large fourth tooth in the lower jaw also fits outside the upper jaw, there is a well-defined constriction in the upper jaw behind the nostrils to accommodate it when the mouth is closed. The fourth tooth of a crocodile sticks out when its mouth is closed.