306 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



dence that birds once had teeth, but they have gradually- 

 dwindled and passed away, because another apparatus 

 — viz., a horny beak — was used in its place. This pre- 

 sumption is confirmed by the finding of Jurassic and 

 Cretaceous birds with the mouth full of teeth — true sock- 

 eted teeth (Fig. 191). These, however, were conical, pre- 

 hensile, not masticatory teeth. 



Reptiles. — There is great variety among reptiles in 

 this regard. This was to be expected, for they are the 

 ancestors of both birds and mammals. Some, as the tur- 

 tles, have no teeth, but, like birds, a horny, nipping beak 

 in their place. Some, like serpents, have teeth not only 

 in the jaws, but on other bones of the mouth, as, e. g., 

 the palatal. But in all reptiles which have teeth these 

 are conical, prehensile, and not masticatory teeth. Also, 

 with the exception of crocodilians, the teeth are not sock- 

 eted. They are formed in a fold of the gum, and after- 

 ward fixed to, but not sunk into and inclosed by the 

 jawbone. Among extinct reptiles socketed teeth were 

 more common. In serpents the teeth all point back- 

 ward. This is necessary in swallowing large prey, which, 

 by a peculiar movableness of the bones of the head, are 

 thus dragged by main force down the throat. 



Fangs of Serpents. — These are worthy of brief 

 notice as an example of admirable adaptive modification. 

 First, observe that the canal in the tooth which conveys 

 the poison does not go to the end, for that would inter- 

 fere with the keenness of the point, but comes out a lit- 

 tle short of the extreme end. The same device is used 

 in the subcutaneous injector of the surgeon. Observe, 

 second, that the formation of the poison canal is not in 

 violation of the ordinary structure of teeth, but a curi- 

 ous modification of it. It is not along the tooth cavity, 

 but is really outside of the tooth. Fig. 192, a, represents 

 a section of a flat tooth. Suppose such a tooth bent 



