190 1.] Gidley, The North American Species of Equus. 99 



(i) The antero-posterior diameters of the grinding surfaces of 

 all the intermediate teeth are greatest at the stage when the tooth 

 has just fully come into use, that is, when about one-half of an 

 inch, or less, of the crown has been worn away; from this point 

 the antero-posterior diameter diminishes very rapidly for a short 

 distance and then continues to diminish more gradually to the 

 roots of the tooth. 



(2) The antero-posterior diameter of the first premolar (p-^) 

 remains about the same for the whole length of the crown, except 

 that sometimes it narrows slightly near the roots. 



(3) The antero-posterior diameter of the last molar (m^), how- 

 ever, is relatively small at first, and increases continually as the 

 tooth is worn away. 



(4) When the teeth first come into use the transverse diameters 

 of all the teeth of the series are quite narrow, owing principally 

 to the rapid incurving of the ectoloph; this diameter increases 

 very rapidly for about one-half to three-fourths of an inch, but 

 from this point to the roots of the teeth the transverse diameters 

 of p-2- to m-^- inclusive remain about the same, diminishing slightly 

 near the roots; p^ gradually diminishes while m-^^ increases in 

 transverse diameter as the crown wears away. 



(5) The antero-posterior diameter of the protocone in all the 

 teeth of the series remains the same for the whole length of the 

 crown. 



(6) The antero-posterior or long diameter of the incisors di- 

 minishes with age while the transverse diameter increases. 



b. Effect of Wear on the Relative Measurements of Tooth Crowns. 



(i.) Ratio of the antero-posterior to the transverse diameter. 



It will be seen from the foregoing that owing to the very slight 

 variation of the transverse diameters of the crowns of p^ to m^ 

 inclusive, for almost their entire length, and to the great shorten- 

 ing of their antero-posterior diameters, the ratio of these diameters 

 in these teeth is very different in old and in young individuals of 

 the same species. Thus in the little worn condition of these 

 teeth in a young horse, especially before the teeth have worn to 

 that stage where the transverse diameter is greatest, the antero- 

 posterior diameter is always greater than the transverse. As the 

 crown wears away, the antero-posterior diameter diminishes and a 



