THE HEAD AND NECK 



125 



The main difference is that, in the short-crowned tooth 

 of Anchitherium, the ridges and valleys are necessarily 

 very shallow, with sloping walls, and there is no need 

 for a packing of cement around and within them ; 

 while in the horse they descend the whole depth of the 

 elongated crown of the tooth, with nearly parallel walls, 

 so that any part presents an almost identical section, and 



Fig. 21. — a, grinding-surface of unworn molar tooth of 

 Anchitherium ; b, corresponding surface of unworn molar 

 of young horse ; c, the same tooth after it has been some 

 time in use. In the latter, the uncoloured portions are 

 the dentine or ivory, the shaded parts the cement filling 

 the cavities and surrounding the exterior. The black 

 line separating these two structures is the enamel, or 

 hardest constituent of the tooth. 



they are filled in and packed round with an abundance 

 of cement. The four original main cusps — antero-in- 

 ternal, posterointernal, antero-external, and postero- 

 external — and also the two intermediate cusps, are dis- 

 tinctly recognisable, but they are prolonged anteriorly 

 and posteriorly into ridges or walls taking a generally 

 crescentic form, with the concavity of the crescent 

 looking outwards, and its convexity inwards. It is this 



