126 



THE STEUCTUEE OE THE HOESE 



disposition which gives the outer surface of the teeth 

 its deeply ridged and grooved or fluted character, the 

 two deep grooves corresponding with the concavities of 

 the two outer crescents. The internal columns, especially 

 the anterior one, have a great tendency to detach them- 

 selves 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 Equidce, and its complete de- 

 tachment in the Hipparion (Hipjootherium) 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. 66). 



Tbe two deep holes, of a roughly crescentic shape, 

 filled with cement, are spoken of, for descriptive purposes, 

 as the anterior and posterior lakes. The sinuosities of 

 their enamel margins, which are sometimes extremely 

 complex, present great variations in different species, as 

 also do the indentations in the edges of the sinus which 

 runs forwards from the inner side of the tooth between 

 the two internal columns, the form of the folds at the 

 bottom of which constitutes the only easily recognisable 

 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 practically 

 the same, the latter are scarcely more than half the 



