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University of California Publications in Geology [Vol. 10 



metaconid-metastylid column and entoconid, and in greater complica- 

 tion of its enamel folds. Of the Great Basin and Pacific Coast province 

 equids Hipparion mohavense callodonte of the Ricardo Pliocene most 

 nearly approaches the form from Ironside. In the West Coast Mery- 



Figs. la to l' 1 . Hipparion cmthonyi Merriam. P;, type specimen, no. 22351, 

 natural size. Fig. la, occlusal view; fig. lb, outer view; fig. lc, inner view. 

 From late Tertiary beds near Ironside, Oregon. 



Fig. 2. Hipparion anthonyi Merriam?. Fragment of an upper cheek-tooth, 

 no. 22355, natural size. From late Tertiary beds near Ironside, Oregon. 



chippus and Pliohippus species the metaconid-metastylid column is 

 relatively shorter anteroposterior^, and wider transversely, and the 

 entoconid is not filled out to the same extent on the antero-internal 

 angle. In general the entoconid of Meryeh ippus and Pliohippus 

 species is truncated obliquely on the antero-internal angle by a plane 

 or curved face, extending outward and forward from the inner side, 

 while in Hipparion this region is expanded and the cross-section of 

 the entoconid tends to take on a rectangular outline, instead of the 

 approximately triangular section seen in M cry chippus, or the tri- 

 angular to imperfectly rectangular but anteroposteriorly short section 

 seen in Pliohippus. 



In no Great Basin or Pacific Coast species of Mcrychippus, Proto- 

 liippus, or Pliohippus known to the writer does the enamel show such 

 a high degree of secondary folding as in the specimen 22351 from Iron- 

 side. In Hipparion mohavense callodonte the enamel folds show much 

 secondary crinkling, though less than in the Ironside form. The crown 

 of H. in. callodonte is slightly larger and relatively thicker trans- 

 versely, and the entoconid fuller on the antero-internal angle. In the 



16 



lc 



2 



