190 1.] Gidley, The North American Species of Equus. 93 



were very carefully and accurately made by Mr. Bruce Horsfali. 

 The photographs were taken by Mr. A. E. Anderson. 



The nomenclature of the elements of the molar teeth employed 

 in this paper is that introduced by Prof. Henry F. Osborn and 

 is clearly indicated especially in Figs, i and 2a, pp. 94, 95. 



I. Principles of Tooth Structure. Characters Due to 



Individual Variation and to Age or Degree of Wear 



Distinguished from truly Specific Characters. 



The following are the chief characters used in definition by 

 Owen, Leidy, and Cope: 



Tooth Characters. 



1. Degree of complexity of enamel foldings (Owen, Leidy, and 



Cope). 



2. Degree of incurvation of superior molars (Owen, Cope). 



3. Degree of curvature of superior molar-premolar series 



(Owen). 



4. Relation of the antero-posterior and transverse diameters of 



the triturating surfaces of the superior molars (Owen, Cope). 



5. Ratio of antero-posterior diameter of protocone to antero- 



posterior diameter of whole triturating surface (Cope). 



6. Comparative size (considered unimportant unless difference 



was considerable) (Owen, Leidy, Cope). 



7. Presence or absence of cup in external lower incisor (Cope). 



8. Relative size of third superior molar (Owen, Cope), 

 g. Crimpled or ptychoid enamel of fossette (Cope). 



10. Character of the separation of the lobes of the metaconid- 

 metastylid columns (Cope). 



Skull Characters Relative to the Teeth. 



11. The position, in relation to the teeth, (a) of the anterior ex- 

 tension of the maxillary ridge, (b) of the anterior extension 

 of the palatal notch, (c) of the post-palatal foramina; also 

 (d) the amount of the posterior projection of the maxillary 

 beyond m- (Cope). 



More importance has been attached to most of these characters, 

 apparently, than their merits warrant; evidently, therefore, the 



