214 



ANIMAL LIFE PAST AND PRESENT. 



and 73 being taken from teeth of opposite sides of the 

 jaw. It is, however, quite evident that the parts 

 marked pa and me in Fig. 72 correspond with the outer 

 wall of Fig. 71 ; while pr and hy in the former repre- 

 sent the inner termination of the transverse crests in 

 the latter. Further comparison of the two figures will 

 convince us that the parts marked pi and pp in Fig. 72 

 are the equivalents of the middle portions of the two 

 transverse crests in Fig. 71. A comparison of Figs. 



FIG. 72. A left upper cheek- 

 tooth of the extinct horse-like 

 Anchithere. (After 0*ln>rii.) 



L''KJ. 7:). Grinding surface of a 

 right upper cheek-tooth of a 

 Horse, p, corresponds with 

 vr, and //// with Jin in Fig. 72. 



72, 73 will show their unity of plan, the central islands 

 in the latter representing the hollows or valleys in 

 Fig. 72, which have now become completely enclosed 

 by the enamel, and are filled by the white cement. 



The height of the crown of one of the grinders of a 

 Horse is, as most of us know, more than three inches 

 and the admirable grinding surfaces formed by alterna- 

 tions of three substances of different hardness, arranged 

 in the complex manner seen in Fig. 73, is self-apparent. 



