Cope-] I" [May 16, 



plates of the dentition of the existing species of Equus*, shows the truth 

 of this statement. Among the extinct species of Equus the range of va- 

 riation is greater. 



The following attempt at a discrimination of the species known to me, 

 or so fully described as to be well known, must necessarily be regarded as 

 provisional, until the skeletons are more fully recovered. American ex- 

 tinct species only are introduced : 



I. Long diameter of anterior internal lobe of superior molars not greater 

 than one third the long diameter of the crown. 



Borders of lakes crenate ; internal anterior lobe notched on the inner side 

 so as to be bilobate ; crowns a little curved ; large E. crenidens. 



II. Long diameter of anterior internal lobe more than one-third and 

 not more than one-half the anteroposterior diameter of the crown. 



a Crowns more or less curved. 



Crowns wider than, or as wide as long ; enamel edges little folded 



E. curvidens. 

 aa Crowns straight or nearly so. 

 /? Diastemata longer. 



Crowns nearly square, enamel not very complex ; no facial fossa ; maxil- 

 lary bone produced much beyond M. iii E. caballus. 



fifi Diastemata shorter. 

 y No facial fossa. 



Crowns nearly square ; enamel not very complex ; maxillary bone little 



produced behind last molar ; smaller 



E. hemionus; E. burclielli; E. quagga; E. zebra ; E. asinus. 



Crowns longer than wide on face ; enamel little complicated ; face and 

 maxillary unknown ; large E. occidentalis. 



Crowns square ; enamel more folded than in other species ; face and max- 

 illary unknown ; large ...E. major. 



r 



A facial fossa. 



Crowns nearly square ; enamel less complex ; maxillary short posteriorly ; 



smaller , E. andium. 



III. Long diameter of anterior inner lobe more than half that of crown 

 of molar teeth. 



Crowns square ; enamel little complex (in Mexican specimens) ; diaste- 

 mata and maxillary behind shorter ; no facial fossa ; large. ..E. excelsus. 



Crowns square ; enamel little complex ; smallest species E. barcencei. 



In using the above table it must be noted that gradations in the diame- 

 ter of the anterior internal column (or lobe) exist, not only between indi- 

 viduals of the same species, but between different teeth in the same jaw. 

 This diameter is always greatest in the last superior molar, and the charac- 

 ters of this tooth are such that they cannot be used in connection with the 

 above table. 



* Philosophical Transactions, 1869. 



