ALLIGATOR, a name originally given by the British Colonists of the Southern States of the North American Union, to a largo species of reptile closely resembling the Crocodile of Egypt, but which modern researches have shown to possess characters generically differing from those of that animal. The word is supposed to be a corruption of an old Indian =ma According to its modern acceptation among zoologists, the name is no longer confined to the species most commonly found in Carolina, Louisiana, and the other Southern States of the Union ; but it is applied generically to all the other American species which agree with it in its most prominent aud influential characters, and which have been called Caymrum, Jazares, kc., by the Spaniards, Portuguese, and Indians of South America. The characters which are proper to the Alligators, and by which they are distinguished from the Crocodiles of the Old World, are by no means of such importance with respect to the influence they may be reasonably supposed to have upon the habits and economy of these animals as to warrant the formation of these reptiles into a distinct and separate genus : their manners and habits are precisely those of the truo crocodiles, and if they differ in certain minor details of structure, this difference should be considered not as a generic charac ter, but as purely specific.
M. Cuvier thus distinguishes the Alligators from the true Crocodiles : "The alligators have the head less oblong than the crocodiles; its length is to its breadth, measured at the articulation of the jaws, as three to two ; the teeth are unequal in length and size ; there aro at least 19, sometimes even as many as 22, on each side in the lower jaw, and 19 or 20 in the upper. The front teeth of the under jaw pierce Ahrough the upper at a certain age, and the fourth from the front, aehith are the longest of all, enter into corresponding holes of the upper jaw,in which they are concealed when the mouth is closed. The hind legs and feet are round and neither fringed nor pectinated on the sides; the toes are not completely webbed, the convecting membrane only extending to their middle ; and finally, the post orbital holes of the cranium, so conspicuous in the true crocodiles, are very minute in the alligators, or even entirely wanting." The Crocodiles, properly so called, on the contrary, have the head at least twice as long as it is broad; 15 teeth on each side of the lower jaw, and 19 on each side of the upper. The incisor or front teeth, as in the alligators, pierce through the upper jaw, at a certain age, but the fourth or largest of the lower jaw, instead of being received into a corresponding hole of the upPeropasaes into a notch on each aide of it; and finally, the hind foet.are bordered by a dentienlated fringe, and the toes are completelyornited 17 a swimming membrane.
The characters here reported as peculiar to the alligators and croco diles respectively, are evidently not of sufficient importance to exert any very sensible influence upon their general economy. Of the „characters and organic modifiostious -which they possess in common, -the principal is the long taper tail, strongly compressed on the sides, and surmounted towards its origin-with a double series of keel-shaped plates, forming two upright denticulated crests, which, gradually con .verging towards tho middle of the tail, there unite and form n single row to the extremity. Its great size, and laterally-compressed form, render the tail an organ of the utmost importance to the crocodiles and alligators : it is true that its weight materially impedes their motions on dry land, but it is a moat powerful instrument of progression in the water, and influences the aquatic habits of these animals much more than their webbed feet. The latter character, indeed, is comparatively of little weight : the hind feet are only used to assist the progression in slow and gentle motion, but in all sudden and violent actions the tail alone is the active instrument ; aud even when the nnimal is surprised on land, as we are assured by Adanson, it becomes n powerful weapon of offence. The compression of the tail is not peculiar among reptiles to crocodiles, though so powerfully influencing their habits ; but the second character which is °amnion to the entire genus, namely, the palmated or semi-palmated hind feet, is exhibited by no other genus of reptiles., though ell are wore or less addicted to an aquatic life. This fact sufficiently demonstrates the small influence which the palmated form of the extremities exerts upon the economy of these animals in general. Still this character is by no means devoid of importance, though in proportion to its utility in aquatic progression it reudera the terrestrial motions of the animals extrernedy slow and awkward ; and this effect is still further increased by the length and weight of the tail at one cud, and by the anatomical structure of the neck at the other. Each of the cervical vertebrx has on either aide a species of false rib, and their meeting at the extremities along the whole neck completely hinders the animal from turning its head to either side, and renders all its movements stiff and constrained. Neither is the pace of the crocodiles on land so swift as to make them objects of fear to ordinary quadrupeds; a man can easily outstrip them, and so sensible are these animals of their own inferiority in this respect, that they immediately retreat to their more congenial element upon the most distant appearance of the human species.