210 THE MAMMALIA. 



It is only in its historical connection that the 

 peculiarity of the horse's dentition acquires a 

 peculiarly significant interest, and as in the case 

 of the three- toed foot when viewed apart from the 

 historical course of its development, seems simply 

 an incomprehensible peculiarity, of no importance 

 either to the horse itself or to the horse fancier, 

 Palaeotherium, Anchitherium, and Hipparion pos- 

 sess, when full grown, seven cheek-teeth above and 



4 3 



below on both sides of the jaw, p -, m - . On the 



other hand the normal formula in the horse's den- 

 tition is p -, m ; it changes only three of its 

 o o 



milk-teeth, and gets three other molars. Now it 

 has long since been known to breeders and veterinary 

 surgeons that, pretty frequently, the horse's row 

 of cheek-teeth begins with one stump too many, 

 the so-called ' wolfs tooth ' (on Owen's drawing 

 marked by the letter p). This most perfectly ex- 

 presses the fact that it occupies the place where, 

 in Palaeotherium among others, we have the first 

 premolar. When it appears in the horse, however, 

 the ' wolf's-tooth ' is not deciduous. It is most 

 obviously a tooth in the last stage of disappear- 

 ance, an irregularly appearing descendant from the 



