Various facts point, thinks Gadow, “to some Theropodous Dinosaurian stock of which the Crocodilia may well form an aquatic, further developed branch” (_Cambridge Natural History_, p. 432). [Illustration: Skull of =Belodon=. A, from above; B, from below. _A_, orbit; _Bo_, basi-occipital; _Ch_, internal nares; _D_, pre-orbital fossa; _Exo._ exoccipital; _Fr._ frontal; _Ju._ jugal; _La._ lacrymal; _Mx._ maxilla; _Na._ nasal; _Pa._ parietal; _Pl._ palatine; _Pmx._ pre-maxilla; _Por._ post-orbital; _Prf._ pre-frontal; _Pt._ pterygoid; _Qu._ quadrate; _S_, lateral temporal fossa; _S′_, superior temporal fossa; _Sq._ squamosal; _Vo._ vomer. (From Zittel.) FIG. A. A TRIASSIC ANCESTOR OF THE CROCODILIA. From Parker & Haswell, _Textbook of Zoölogy_.] The direct ancestors of the Crocodilia, Gadow says, are still unknown. GEOGRAPHICAL DISTRIBUTION As will be seen by examination of the table (p. 2) from Ditmars, and of Figure 1, the recent Crocodilia are found in all of the great continental areas except Europe; mainly in the tropical or subtropical regions. [Illustration: FIG. 1. MAP SHOWING PRESENT DISTRIBUTION OF CROCODILIA. (After Gadow.)] The alligator is found in the southwestern United States and in China. The crocodile is the most numerous in species and is the most widely distributed of the group. It is especially characteristic of Africa and Madagascar, but is found also in Florida, Mexico, Central and South America, the West Indies, South Asia, the East Indies, and Australia. The gavial is found in India and some of the islands of the Orient, especially Borneo and Sumatra. The caiman occurs in southern Mexico, Central and South America. [Illustration: FIG. 2. HEADS OF AMERICAN ALLIGATOR AND AMERICAN CROCODILE; ALLIGATOR ON LEFT. (After Ditmars.) (Reproduced by Permission of Sturgis & Walton Co.)] The distribution of individual forms will be mentioned again when they are discussed in detail. ALLIGATOR MISSISSIPPIENSIS Since this animal, generally known as the American or the Florida alligator (formerly _A. lucius_), is the one upon which most of the facts of this book are based, it will be discussed first. At this point it may be well to answer the question that is sure to be asked by someone early in any conversation upon the Crocodilia. The writer, and doubtless every other zoölogist, has been asked countless times, “What is the difference between an alligator and a crocodile?” As a matter of fact there is, perhaps, no absolute distinction between the two groups, but there are certain features that make it easy to distinguish, say, between the American alligator and the American crocodile.