In this sub-order are all the living members of the Crocodilia, distinguished by having the bodies of the dorsal vertebrae concave in front (procoelous). Three distinct types may be distinguished amongst the living Crocodilia. The Gavial is distinguished by its elongated snout, at the extremity of which the nostril is placed, and by the fact that the teeth are pretty nearly equal in size and similar in form in the two jaws. In the true Crocodiles (fig. 310) the fourth tooth in the lower jaw is larger than the others, and forms a canine tooth, which is received into a notch excavated in the side of the alveolar border of the upper jaw, so that it is visible externally when the mouth is closed. In the Caimans or Alligators the same tooth in the lower jaw forms a canine, but it is received into a pit in the palatal surface of the upper jaw, where it is entirely concealed when the mouth is shut. The Crocodiles have the hind-legs bordered by a toothed fringe, and the toes completely united by membrane. They are essentially natives of fresh water, but sometimes frequent the mouths of rivers. They occur chiefly in Asia and Africa, but species are found in some of the West Indian Islands. The Alligators have the hindlegs simply rounded, and the feet not completely webbed. They are essentially aquatic, and are voracious animals, living upon fish or Mammals. The best-known species are the Alligator of the southern United States (A. Mississippiensis), the Caiman (A. palpebrosus) of Surinam and Guiana, and the "Jacare" or Spectacled Alligator (A. sclerops) of Brazil. The Gavials inhabit fresh waters, and appear to be exclusively confined to the Ganges and other large rivers of India. The Gangetic form (Gavialis Gangeticus), in spite of its numerous pointed teeth, is not so highly carnivorous as the true Crocodiles.
Fig. 310. - Skull of the Crocodile.
True procoelian Crocodiles occur for the first time in the Greensand (Cretaceous series) of North America. In Europe, however, the earliest remains of procoelian Crocodiles are from the Lower Tertiary rocks (Eocene). It is a curious fact that in the Eocene rocks of the south-west of England, there occur fossil remains of all the three living types of the Croco-dilia - namely, the Gavials, true Crocodiles, and Alligators; though at the present day these forms are all geographically restricted in their range, and are very partially associated together.
The Amphicoelian Crocodiles, with biconcave vertebrae, are entirely extinct. They have but a limited geological range, extending only from the Trias to the Chalk inclusive, and being therefore strictly Mesozoic. The biconcave vertebrae show a decided approach to the structure of the backbone in fishes; and as the rocks in which they occur are marine, there can be little doubt but that these Crocodiles were, in the majority of cases at any rate, marine. The most important genera belonging to this order are Teleosaurus, Belodon, Stagonolepis, Steneosaurus, Dakosaurus, Makrospondylus, and Suchosaurus, the last being from the fresh-water deposits of the Wealden (Cretaceous).