126 Bulletin American Museum of Natural History. [Vol. XIV, 



character of " the relatively great width of the metaconid-metasty- 

 lid column of the inferior molars " is only a feature of individual 

 variability, as this column is very variable in size. The charac- 

 ters that seem to point most strongly to the association of this 

 species with the genus Hippai'ion are as follows: (i) The appres- 

 sion of the metaconid-metastylid column to the protoconid and 



Fig. 14. Hipjtarion eu- 

 rystylus. Type of Equus 

 e»rysiyh(s Cope. Superior 

 premolar. (After Cope.) 



Fig. 15. Hipfiarion eu- 

 rystylus (Equus eurysty- 

 hts Cope). Lower molar. 

 (After Cope.) 



Fig. 16. Hipparion eu- 

 ryityljis {Equus eury- 

 siy/us Cope). Lower pre- 

 molar. (After Cope.) 



hypoconid, which seems to be characteristic of Protohippus and 

 Hipparion and not of Equus ; (2) the presence of a well-defined 

 enamel keel or loop in the anterior border of the protoconid; (3) 

 the greater extension of the transverse ridge of enamel thrown 

 out from the parastylid, which extends beyond the middle and 

 usually to the outer edge of the metaconid, and marks the antero- 

 internal corner of the tooth in E. eury stylus and all the three-toed 

 horses; and which in E. caballus and the fossil species of Equus 

 seems seldom to extend beyond the middle of the metaconid and 

 often only as far as its inner margin: 



(i) The tendency toward fiat external faces of the protoconid 

 and hypoconid, and (2) toward the formation of the external 

 median keel, mentioned by Cope, together with its size, seem 

 definitely to place this species in the genus Hipparion. (Compare 

 Cope's figures of E. eurystylus with Leidy's figures given in ' Ex- 

 tinct Mammalia of Dakota and Nebraska,' PI. xix, figures 4, 

 6, 10, 31, 34, 37, and 40.) The specific distinctions apparently 

 cannot be made out, 



(18) Protohippus cumminsii {Cope). 



Equus cumminsii Cope. 

 Type Locality. — Mt. Blanco, Texas. 

 Horizon. — Blanco Beds. 

 Type. — Superior molar. 



Author's description. — "The enamel borders are entirely simple. . . . 

 It differs from the E. simplicidnis and E. tan in the prominent concavity of the 



