196 WATER REPTILES OF THE PAST AND PRESENT 



in swimming, its breadth being increased by a vertical row of horny 

 plates above. The limbs are of the ordinary elongated type — 

 ambulatory rather than swimming legs — and are not of much use 

 for propelling the body in the water; the front legs indeed are 

 usually held close to the body while the animal is swimming. 

 The toes, however, are partially connected by webs, to a slight 

 extent only in the alligators and crocodiles, but much more so in 

 the long-snouted gavials. The feet have five toes in front and 

 four behind; and the loss of the fifth toe can only be ascribed 

 to terrestrial habits. The body is covered more or less with horny 

 scutes or scales, beneath which are several rows of thickened, 

 pitted, bony plates on the dorsal side, and sometimes also on the 

 under side, forming a more or less extensive bony armor. The 

 eyes have movable lids, as in most lizards, and the ear-opening is 

 small. 



But the external appearance of these reptiles is not sufficient 

 to distinguish them widely from other groups, and we must resort 

 to the internal structure, especially that of the skeleton, for the 

 more essential differential characters. The most crucial of these, 

 the one which more than any other determines their relationships, 

 consists in the position and shape of the bone with which the lower 

 jaw articulates, the quadrate bone, so characteristic of reptiles. 

 As in the plesiosaurian and ichthyosaurian skulls, it is firmly united 

 with the adjoining bones, not articulating freely with them, as in 

 the lizards and snakes. But this fixed relation of the bones is very 

 unlike that of the plesiosaurs, ichthyosaurs, and turtles, in that it 

 is held in place by two bony bars or arches, the upper extending 

 forward to unite with the bones at the back part of the orbit, the 

 lower, with the hind extremity of the upper jaw. The lower jaws 

 are rigidly united in front, sometimes for a long distance; they have, 

 almost always, a hole or opening through the hinder part, known 

 in but few other reptiles. The bones of the palate are all firmly 

 united, forming a nearly complete roof, very unlike the condition 

 in the mosasaurs and lizards. The palate also is very peculiar 

 in the development of a plate of bone below the nasal chambers, 

 forming a complete bony canal on each side through which the 

 respiratory air passes far back to the internal opening of the nostrils 



