FOSSIL HORSE REMAINS FROM IDAHO — GAZIN 305 



enamel walls. The reentrant between the outer columns is shortest 

 in Dp2 and deepest in Dp4, with an ectostyUd usually best developed 

 in Dp2. In each dentition the metaconid increases in length antero- 

 posteriorly in succeeding teeth and the me tasty lid decreases. The 

 groove between these columns in most dentitions is shallow though 

 somewhat acute. The two Ungual reentrants are transversely com- 

 pressed though of considerable anteroposterior extent. A marked 

 feature of the milk premolars, more noticeable in the fossil material 

 than in recent jaws, is the development of slender styles trans- 

 versely along the anterior and posterior walls of the teeth (fig. 22, 

 no. 12560), except in relatively little worn dentitions. The para- 

 stylid and an antero-external style usually extend the width of the 

 tooth in the fossil Dpa and Dp4, and the hypoconuHd combined 

 with a hypostylid extends an enamel surface across the posterior 

 wall of Dp2 and Dps. The parastylid of Dp2 and the hypoconuHd of 

 Dp4 are normally extended fore and aft. 



Hyoid. — Parts of the hyoid have been found associated with the 

 skull in several cases, and in a few the assembly is well preserved 

 and nearly complete (pi. 29). The stylohyal is about the length 

 of that in E. grevyi, as represented in two specimens, but is deeper 

 and more robust, particularly through the middle section. This 

 element is similar to that in E. caballus but less variable in size, 

 and the muscular or postero ventral angle is well rounded though 

 less attenuated than in the few recent specimens observed. Also 

 the process that connects with the tympanohyal is less drawn out. 



The epihyals were not found in any of the specimens, and the 

 ceratohjT^als are not very well preserved, but these appear proportioned 

 about as in the Arabian horse, not so heavy as in some of the larger 

 domestic forms. The basihyal is moderately robust with a prominent 

 Ungual or glossohyal process wliich shows a more marked concavity 

 or groove along each side than in any recent material examined. A 

 single basihyal of E. grevyi shows a much shortened lingual process. 

 The tliyrohyal is variable in length and shows a prominent groove 

 along the ventromedial surface extending from the basihyal well 

 toward the posterior extremity. Tliis portion of the thyrohyal 

 examined in Equus caballus is conspicuously flattened ventrally. 



Vertebrae. — The cervical series (pi. 31, fig. 1) is about as long as 

 in the Arabian horse, but in most cases somewhat slenderer. Com- 

 pared with the large zebra the neck is a little longer and relatively 

 perhaps a little slenderer. Except for less width across the posterior 

 articular surfaces, the atlas shows no significant characters dis- 

 tinguishing it from that of the recent horse. The neural spine of the 

 axis is distinctly less outstanding and less convex in the fossil form 

 than in either E. caballus or E. grevyi, and the width of the anterior 

 articular surface for the atlas is less than in E. caballus. In the 



