134 THE STRUCTURE OF THE HORSE. 



of young animals, and deserve careful study, on ac- 

 count of the light they throw upon the structure of 

 the organ in its maturity. It will be seen (Fig. 21) 

 that the pattern presented by the free surface is es- 

 sentially similar to that of the corresponding tooth 

 of Anchitherium, which is itself a modification of 

 that of Hyracotherium, from which it is but an easy 

 transition to Phenacodus, as shown in the first chap- 

 ter. The main difference is that, in the short-crowned 

 tooth of Anchitherium, the ridges and valleys are nec- 

 essarily 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 they are filled in and 

 packed round with an abundance of cement. The 

 four original main cusps antero-internal, postero- 

 internal, antero-external, and postero-external 

 and also the two intermediate cusps are distinctly 

 recognizable, 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 disposition which gives the outer surface of the 

 teeth its deeply ridged and grooved or fluted charac- 

 ter, the two deep grooves corresponding with the 



