68 THE PERMANENT DENTITION. 



and paleothere.* The anterior valley between the nar- 

 row ridge and the first principal internal column ex- 

 pands into a subcrescentic fold. The second is a short, 

 simple fold, and terminates opposite that which pene- 

 trates the tooth from the outer side. The third inner 

 fold expands in the posterior lobe of the tooth hke the 

 first, and two short folds partially detach a small ac- 

 cessory lobe at the posterior part of the crown. All 

 the valleys, fissures, or folds, in both the upper and the 

 lower grinders, are lined by enamel, which also coats 

 the whole exterior surface of the crown. 



" The character by wdiich horses' grinders may best 

 be distinguished from the teeth of other herbivora cor- 

 responding with them in size, is the great length of 

 the tooth before it divides into roots. This division, 

 indeed, does not begin to take place until much of 

 the crown has been worn away. Thus, except in. old 

 horses, a considerable proportion of the whole of the 

 tooth is implanted in the socket by an undivided base. 

 This is slightly curved in the upper grinders. 



"The deciduous molars have shorter bodies than the 

 permanent, and sooner begin to develop roots. They 

 may be distinguished from the rooted molar of a rumi- 

 nant, as may also their permanent successors with 

 roots, by their form and the pattern of their grinding 

 surface. The latter may be a little changed by the 

 partial obliteration of its enamel folds, but it gen- 

 erally retains enough of its character to show the 

 distinction." 



* " The species of paleotlierium, which appear to have accom- 

 panied the anoplotheres in the first introduction of hoofed quad- 

 rupeds upon this planet, were characterized by the same com- 

 plete dental formula, namely, forty-four functionally developed 

 teeth." — Owen. 



