The crocodiles, alligators, caimans and the gavial make up a group of about twenty reptiles. They all have the same basic appearance and, like the turtles and terrapins, they are all amphibious. The alligator differs from the crocodile in having shorter and broader snout although there is yet another, more definite difference. In both animals the fourth tooth of the lower jaw is enlarged. The teeth in the narrower upper and lower jaws of a crocodile are in line and so this tooth fits into a groove of the upper jaw, and can be seen when the crocodile closes its mouth. The teeth in the broader, upper jaw of an alligator overlap those in the lower. The enlarged tooth this time fits into a pocket in the upper jaw and cannot be seen when the mouth is closed.
The long, powerful tail of the crocodile indicates that it is a good swimmer. The ears, eyes and nostrils are all in line on top of the head so that it can lie practically submerged waiting for its prey. It is also quite agile on land and leaves the water to breed and bask in the sun.