DAKOTA AND NEBRASKA. 281 



HiPPARION OCCIDENTALE. 



In the rich collection of fossils obtained by Dr. Hayden from the Maiivaises Terres 

 of White River, Dakota, and now in possession of this Academy, there are five teeth 

 of a species of Hipparion, which has been distinguished by the above name. The 

 specimens are black, and well preserved. They have no adherent matrix, and are 

 very different in appearance from the ordinary Mauvaises Terres fossils. According 

 to Dr. Hayden, they belong to the superficial portion of the tertiary deposits, or bed 

 F of the section, as indicated on page 16, and which is more fully developed on 

 the Niobrara River and at Bijou Hill. 



The teeth consist of four upper molars of the right, and one of the left side, and 

 are represented in figures 1 — 5, plate XVIII. They were accompanied by a fragment 

 of a last upper molar and a lower molar, apparently of another equine animal. 



The specimens are between a third and a half worn away, and exhibit on their 

 triturating surface the characters usually ascribed to the genus. They indicate a 

 much larger animal than the Ilipparion vennstum of South Carolina, and approach 

 in size those of H. gracile of Europe. They have nearly the size, proportions, and 

 curvature of the teeth of the Ass. 



In the upper molars, the course of the enamel, as seen on the masticating surfaces, 

 figures 1 — 5, is not so complex as in H. gracile or H. mediterraneum, but is sufficiently 

 so to exhibit an evident relationship. 



The antero-iuternal column of the crown is of uniform width throughout its length, 

 except in the first molar (4), in which it feebly widens near the bottom. In trans- 

 verse section, in the anterior molars, it is reniform ; in the fourth (2) and fifth (1) 

 molars elliptical. The external buttress-hke ridges of the crown in the anterior 

 molars are as robust as in the Ass, and the concave intervals are comparatively deep. 

 The posterior valley, or inflection of enamel, extends to near the bottom of the 

 crown. 



The enamel is observed on the masticating surface to be complexly folded at the 

 sides of the central lakes, except along the back border of the posterior lake. The 

 median fold of the inner enamel border of the crown, extending towards the isolated 

 reniform lake, is trilobate in the first molar (4) and the third (5) of the left side, but 

 it is bilobate in the second (3) of the right side ; and in both the latter it is deeply 

 inflected at its back part, so as even to reach the centre of the masticating surface. 

 In the fourth (2) and fifth (1) molars the median fold just mentioned is compara- 

 tively simple. 



The measurements of the specimens of superior molars are as follow : 



36 



