Wikipedia: Alligator

Public domain picture from U.S. Fish and Wildlife Service [full size picture]

Kingdom Animalia, Phylum Chordata, Class Reptilia, Order Crocodilia?

It differs from the true crocodile principally in having the head broader and shorter, and the snout more obtuse; in having the fourth, enlarged tooth of the under jaw received, not into an external notch, but into a pit formed for it within the upper one; in wanting a jagged fringe which appears on the hind legs and feet of the crocodile; and in having the toes of the hind feet webbed not more than half way to the tips.

Alligators proper occur in the fluviatile deposits of the age of the Upper Chalk in Europe, where they did not die out until the Pliocene age.

They are now restricted to two species, A. mississippiensis or lucius in the southern states of North America up to 12 ft. in length, and the small A. sinensis in the Yang-tse-kiang.

In Central and South America alligators are represented by five species of the genus Caiman, which differs from Alligator by the absence of a bony septum between the nostrils, and the ventral armour is composed of overlapping bony scutes, each of which is formed of two parts united by a suture. C. sclerops, the spectacled alligator, has the widest distribution, from southern Mexico to the northern half of Argentina, and grows to a bulky size. The largest, attaining an enormous bulk and a length of 20 ft., is the C. niger, the jacare-assu or large caiman of the Amazons.

Some crocodiles can be found in salty water, but most alligators stay in fresh water.

Initial text from 1911 encyclopedia