94 Bulletin American Museum of N'atural History. [Vol. XIV, 



only way of forming an estimate of their true value is to find out, 

 in'a large series of specimens, in what way and how much these 

 characters may be affected in the individual and in different in- 

 dividuals of the same species. 



The conditions which affect the tooth character of the Horse 

 may be classed under two heads: (A) Age and (B) Individual 

 Variability. ' 



A. — Tooth Characters as Altered by Age or Degree of 



Wear. 



(I). Effect of I Fear on Complexity of Enamel Folding. 



When a molar or premolar tooth first comes into use, the face, 

 as well as the sides of the crown, is completely covered with 



paraMijle 



hypostyle 



p^'otolopJi— 



enamel (Fig. 2, A), 

 which folds in and 

 out, and (though 

 somewhat hidden by 

 cement) presents the 

 same general appear- 

 ance as that seen 

 in the much more 

 -metfiiojyh primitive forms, An- 

 chitherium and Me- 

 sohippus. Soon the 

 enamel on the tips of 

 the cones and along 

 the ectoloph wears 

 through, and small 

 patches and ridges of 

 dentine completely 

 surrounded by a bor- 

 der of enamel are ex- 

 posed. (Fig. 2, B.^ 



Fig. I. Molars of Horses showing terminology. A, As the tOOth is further 

 Upper molar, Anchitherium. B^ Lower molar, Merychtppus. 



(After.Osborn.) wom away, these 



patches and irregular ridges broaden and rapidly lengthen until 

 when about one-half to three-quarters of an inch of the crown 



'Sex seems to affect the tooth characters only in the canines, which are always prominent 

 m the adult males, but absent or at best vestigial in the females ; hence this factor is left out 

 of the discussion. 



hypohyjgliid 



pai'aKtijUd 



vietastiiUd 



entosfyJid 



