The families and genera of the order Crocodylia are differentiated primarily by the anatomical peculiarities of their skulls. The classification of each species is based mainly upon external characteristics, such as the proportions of the snout, the bony structures on the dorsal side of the snout, the number of teeth, the number and arrangement of the large knobs on the nape of the neck, and the characteristics of the dorsal plates. For instance, alligators possess a broad, U-shaped snout and have an “overbite”—that is, all the teeth of the lower jaw fit within (are closer to the tongue than) the teeth of the upper jaw, whereas crocodiles have a narrow, V-shaped snout. The large fourth tooth on each side of the alligator’s lower jaw fits into a socket in the upper jaw; typically, no lower teeth are visible when the mouth is closed. In contrast, the large fourth tooth on each side of the crocodile’s lower jaw projects outside the snout when the mouth is closed.
http://www.britannica.com/EBchecked/topic/143679/crocodile/38427/Distinguishing-taxonomic-features |