THE HEAD AND NECK. 135 



concavities of the two outer crescents. The internal 

 columns, especially the anterior one, have a great 

 tendency to detach themselves from their crescents 

 formed by the intermediate cusps. The amount of 

 detachment and the form of -this column in section 

 are important in determining the various species of 

 fossil jEquidm, and its complete detachment in the 

 Hipparion (Hippotherium) forms one of the principal 

 characters (already alluded to) by which that genus 

 is distinguished from the other members of the 

 group (see Fig. 10, p. 71). 



The two deep holes, of a roughly crescentic shape, 

 filled with cement, are spoken of, for descriptive pur- 

 poses, as the anterior and posterior lakes. The sinu- 

 osities of their enamel margins, which are sometimes 

 extremely complex, present great variations in differ- 

 ent species, as also do the indentations in the edges 

 of the sinus which runs forwards from the inner side 

 of the tooth between the tw r o internal columns, the 

 form of the folds at the bottom of which constitutes 

 the only easily recognizable distinction between the 

 molar tooth of the common horse and the ass. 



Though the length of the combined grinding-sur- 

 faces of the upper and lower molar teeth is practi- 

 cally the same, the latter are scarcely more than half 

 the width from side to side, and present quite a dif- 

 ferent pattern. As in the upper series, those placed 

 10 



