their most obvious external differences from alligators are visible in the head. crocodiles have narrower and longer heads, and more v-shaped than u-shaped snouts. the alligator's upper jaw is wider than its lower jaw, and the teeth in the lower jaw fit into small depressions in the upper jaw. the upper and lower jaws of the crocodiles are the same width, and teeth in the lower jaw fall along the edge or outside the upper jaw when the mouth is closed. when the crocodile's mouth is closed, the large fourth tooth in the lower jaw fits into a constriction in the upper jaw. for hard-to-distinguish specimens, the protruding tooth is the most reliable feature to define a species.