FOSSIL HORSE REMAINS FROM IDAHO — GAZIN 301 



The protocone on Dp^ and Dp^ though large is usually simple and 

 rounded, resembling tliis cusp in P^. In Dp* this cusp or column 

 appears more advanced showing a more angular antero-internal ex- 

 tremity, a greater anteroposterior extent, and a flatter inner wall. 

 With wear the protocone in each case becomes more nearly oval and 

 broadly connected to the protoloph. 



The hypocone is small in comparison with that in the permanent 

 teeth, and the valley defining it posteriorly is somewhat irregular but 

 reduced with wear much earlier than that posterior to the protocone. 

 The hypostyle generally extends a strong plication lingually, and in 

 one or two instances the valley or reentrant posterior to the hypocone 

 is closed posteriorly, leaving, as in some of the permanent premolars, 

 an isolated lake. More commonly such an isolated lake is formed in 

 early wear near the anterior end of Dp^ similar to that in P^. 



Mandible. — The mandibles (pis. 27, 28) vary appreciably in size 

 and shape, although certain features are observed that are distinc- 

 tive or that show an average difference from mandibles of modern 

 types. The portion of the jaw below the dentition is relatively deep 

 and heavy in young adults, and the line of the occlusal surface of the 

 cheek teeth is more convergent anteriorly with the general direction 

 of the inferior margin of the ramus. The ventral margin shows a 

 pronounced vascular impression anterior to the angle, which is per- 

 haps not so sharply deflected at this point as in E. grevyi but anterior 

 to this point, and centered approximately below P4 is a very marked 

 ventral convexity, which in most cases is distinctly more prominent 

 than in E. grevyi and much more so than in E. caballus. In young 

 individuals the inferior margin of the ramus is more nearly straight 

 (pi. 27, fig. 1, no. 12560). From the ventral convexity forward in 

 adults the lower margin of the jaw rises more rapidly than in E. 

 grevyi to the symphysial portion, which with the incisors is broader 

 and generally liigher with respect to the line of the cheek teeth. The 

 distance between the incisors and the cheek teeth is great though 

 averaging somewhat less than in E. grevyi. The portion of the fused 

 mandibles between the incisors and premolars is constricted dorsally 

 to about the extent in E. grevyi, but perhaps more abruptly so 

 immediately behind the canines (pi. 28). 



The ascending ramus is directed backward about as in E. grevyi 

 with the posterior margin below the condyle nearly vertical (pi. 27, 

 fi-g. 2, no. 12553) with respect to the ventral plane or table surface 

 when resting normally. In E. caballus the anterior margin tends to 

 be more nearly vertical, and the posterior margin rises upward and 

 generally forward to the condyle. The anterior surface of the ascend- 

 ing^ramus appears wider in the fossil jaws than in E. grevyi and the 

 fossa on the upper portion of the outer surface of the ascending ramus 

 appears better defined than in the recent zebra and horse. 



