JAWS 89 



from the head as tlieir bulky prey passes into the 

 throat (Fig. 51). Here also the two halves of the jaw 

 never fuse together in front, but are tied together 

 merely by an elastic ligament which allows them to 

 spread apart in swallowing large objects. Again, this 

 and the double hinge at the skull allows the jaw to 

 be thrust forward, lirst that on one side then on the 

 other, so that the mouth is thus worked over the prey 

 by the backward -curved teeth — one side holding what 

 is gained while the other advances. The snake thus 

 literally gets over (or '' outside of ") its prey. 



There was an old fossil monster called Mosasaiiriis 

 (which was a lizard, but quite serpentlike) that had 

 a better arrangement still. In the middle of each 

 side of the lower jaw was a joint bending downward 

 and outward. On the front part were backward-set 

 teeth. Its jaws also were capable of moving first one 

 side then the other ; but you can see that every time 

 it bit its prey the joints straightened like a nearly 

 open jack-knife and pushed the front part forward 

 by the pressure of the bite. 



Something similar to this, though not just like it, 

 is found now in the upper jaws of those poisonous 

 serpents which have fangs that lie down when the 

 mouth is closed but are erect when the mouth is 

 open. By means of a joint in the middle of the 

 upper jaw (which is pulled straight by the muscles 

 as the mouth opens), the bone lying across the upper 

 end of the mouth, to which the fangs ai*e fastened, is 

 rolled downward and forward, thus letting down the 

 deadly fangs. 



