146 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



As may be supposed, the teeth of the crocodile are of 

 great size and strength. 



An instructive example of a quasi- edentulous con- 

 dition is found in the lizard of New Zealand, which is 

 known as Hatter ia ; the teeth at the sides of the jaw 

 are not replaced when they are worn down, but the 

 bone itself, which is exceedingly dense, takes on the 

 function of a cutting organ. In. forms which are perma- 

 nently edentulous, like the tortoise or the pigeon, the 

 edges of the jaws become invested in horn, the shape 

 and form of which varies with the habits of the 

 animal ; in some birds (wild duck) the edges of the 

 horny case become serrated and give rise to the 

 appearance of tooth- like structures ; in some cases 

 (Odontopteryx) the edge of the underlying bone be- 

 comes denticulated ; these are adaptations to modes of 

 life, and must be carefully distinguished from the 

 actual possession of true teeth such as characterises a 

 large group of extinct birds (Odontoriiithes). 



Curiously resembling the arrangement of the turtle, 

 and having, of course, much the same function, are 

 the tough horny plates oil the jaws of tadpoles ; the 

 history of these plates would, however, seem to be very 

 different from that of the similarly disposed parts in 

 the higher forms ; that is to say, the beak of the 

 tadpole, and, doubtless, the horny apparatus of the 

 lamprey, are structures which preceded, and not suc- 

 ceeded, the possession of teeth. 



A phenomenon similar to that seen in Lophius is 

 to be observed in Snakes ; here, again, the organs of 

 prehension being absent, owing to the disappearance 

 of the limbs (see page 96), the teeth are directed back- 

 wards ; when, therefore, living prey, such as a frog, has 

 entered into the cavity of the mouth, it is prevented 

 from escaping out of it by the erection of the teeth. 

 Some snakes kill their food by constriction, and swal- 

 low it at leisure ; others swallow it whole, and in them 



