mouth at the front end. This peculiar arrangement of the air-passages, so like that of ourselves, has much to do with the water habits of the crocodiles, as we shall see. The teeth are conical in shape, and are always restricted to the edges of the jaws, above and below. They are inserted deeply and firmly in sockets, and are replaced frequently by new ones growing beneath them, pushing the older ones out as their usefulness becomes impaired by injury or by use. In some species there are as many as thirty teeth in each side of the jaws, above and below, although the broad-headed kinds have a smaller number. [Illustration: FIG. 99.] [Illustration: FIG. 100. FIG. 99.—Skull of _Alligator mississippiensis_, from below. FIG. 100.—The same, from above: _bo_, basioccipital; _bs_, basisphenoid; _f_, frontal; _j_, jugal; _l_, lacrimal; _m_, maxilla; _n_, nasal; _p_, parietal; _pa_, palatine; _pm_, premaxilla; _pf_, prefrontal; _pr_, postfrontal; _pt_, pterygoid; _q_, quadrate; _qj_, quadratojugal; _tr_, transverse.] The neck is short, as has been stated, but it always includes in living forms nine vertebrae, a number probably slightly in excess of that of their terrestrial forbears. By the peculiar mode of attachment of the short “hatchet-shaped” ribs, much lateral movement of the neck is prohibited, nor is any very great vertical movement possible. The vertebrae of the whole column, save the atlas, the second sacral, and the first caudal—which is a very remarkable anomaly—are concave in front and convex behind, agreeing in this respect with those of all other living reptiles, save the turtles, the tuatera, and some lizards. The ribs of the neck have their two heads attached, one to the body of the vertebra, the other to the arch, but those of the dorsal region, though double-headed, have both become attached to the transverse projection of the arch, a seemingly trivial character, but one which immediately distinguishes all crocodiles from all other water reptiles, and from all terrestrial reptiles, indeed, save the Parasuchia, Pterosauria, and Dinosauria. The pelvis is firmly attached to the spinal column by two sacral vertebrae. [Illustration: FIG. 101.—Vertebrae of gavial from the side (cervical), and from in front (dorsal): _az_, anterior zygapophysis; _pz_, posterior zygapophysis; _d_, diapophysis; _r_, cervical rib; _c_, articulation for head; _t_, for tubercle of dorsal rib.]