Crocodilians.

The Crocodilia is an ancient class that has existed unchanged for about 200 million years. (a) Alligators, such as this American alligator (Alligator mississippiensis), have a broad snout, and the lower jaw teeth close on the inside of the upper jaw (and thus are almost completely hidden when the mouth is closed). (b) Crocodiles, including this American crocodile (Crocodylus acutus), have a longer, thinner snout, and the lower jaw teeth close on the outside of the upper jaw (and thus are visible when the mouth is closed).