THE BEAD AND NECK 



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being formed of enamel. When the ridges became 

 worn down by the friction of hard particles of food 

 interposed between the opposing teeth, the dentine of 

 the interior was exposed, forming islands surrounded 

 by enamel. As the wear continued and reached the 

 bottom of the valleys, all the enamel coating disappeared 



FlG. 20. — a, lower molar of Anchitherium (brachydont 

 form) ; b, lower molar of a young horse, with the crown 

 slightly worn and the roots not yet formed (hypsodont 

 form) ; c, the same tooth of an old horse, with the crown 

 almost entirely worn away and the roots fully formed. 



from the upper surface, and nothing remained but a 

 plane surface of comparatively soft dentine, surrounded 

 at the circumference by the enamel. With the progress 

 of time, however, individual succeeded individual, in 

 each of which, probably by insensible degrees, the 

 crowns of the teeth became longer, the valleys deeper, 



