CENOZOIC ERA 199 



anterior part of the skull itself. The portion of the skull anterior to the 

 eyes has been elongated into the well-known muzzle (Fig. 10.3D). One 

 result of this elongation has been the production of a gap in the tooth 

 row between the incisor (front) teeth and the grinding battery composed of 



FIG. 10.3. Series of horse skulls in ascending geologic order. A, 

 Hyracotherium (Eohippus). B, Mesohippus. C, Merychippus (Proto- 

 hippus). D, Equus. (From Scott, A History of Land Mammals in 

 the Western Hemisphere, p. 283. Copyright 1937 by American 

 Philosophical Society. Used by permission of The Macmillan Com- 

 pany, publishers.) 



premolars and molars. This toothless gap is called the diastema (Fig. 

 10.3D). It is utilized by man as a convenient location for the bit used to 

 control the movements of the horse. 



The grinding battery composed of premolar and molar teeth is an 



