In the Alligatoridae, the teeth of the lower jaw fit inside those of the upper jaw, whereas in the Crocodylidae, the teeth of the two jaws form a single interdititating row when the jaws are closed.
In crocodiles, the fourth tooth of the lower jaw fits into an indentation of the upper jaw and is exposed to view when the mouth is closed.
In alligators, this tooth is hidden from view when the mouth is closed, because it fits into a pit which exists in the upper jaw.
Other physical characteristics exist; however, the foregoing information is the most outstanding.
Crocodilians are well-adapted as predators, with few natural enemies. Bony plates, called osteoderms, form a kind of armor in their thick skin.
Their teeth, about 30 to 40 in each jaw, are set into sockets in the jawbones and interlock when the mouth is closed.
Crocodilians are the most vocal reptiles, producing sounds from quiet hisses to fearsome roars and bellows, usually during the mating season. On land, crocodilians move quickly in a belly crawl but can also gallop and walk mammal-like on all four legs.
Darrell's friend told Derrick that an easy way to tell the difference between an alligator and a crocodile is that an alligator has an "A" shaped snout and the snout of a crocodile is more rounded.