Crocodylidae includes all 'true' crocodiles and is differentiated from other crocodillian families because their teeth are visibile when the jaw is closed. The fourth tooth of the bottom jaw, the largest tooth in all crocodilians, settles itself in a crease on the outside of the upper jaw, rather than a sheath like that of Alligatoridae. It includes the genera Crocodylus, Osteolamus, and Mecistops.