DAKOTA AND NEBRASKA. 283 



the same arrangement of the enamel as in the former specunon. The less imperfect 

 tooth, probably a fourth molar, measures sixteen lines along the median ridge extei-- 

 nally, and ten lines along the internal column. The antero-postcrior diameter of 

 the triturating surface is nine and three-fourths lines, and the transverse diameter 

 nine and a half lines. The remaining specimen, a mere fragment, belonged to a 

 smaller tooth, probably a fifth of the series. 



Another specimen, probably belonging to H. speciosum, is the outer portion of an 

 upper molar tooth, also a fourth or fifth of the series, which is of especial interest 

 from its having been discovered in Washington Co., Texas, — a locality remote from 

 that in which the former specimens were obtained. It was submitted to my exami- 

 nation by Dr. B. F. Shumard, of St. Louis, Missouri, who informed me that it was 

 obtained, in digging a well, from a white calcareous sandstone of medial tertiary age, 

 at the depth of forty feet below the surface. 



The fragment bears a sufficiently near resemblance to the corresponding portion of 

 the teeth above described in size, proportions, and appearance of the triturating sur- 

 face, as represented in figure 17, to render it probable that it belonged to the same 

 species oi Hipparion. 



From the Niobrara River, Dr. Hayden obtained the following specimens, which 

 px'obably belong to nipparion speciosum, although they present considerable variation 

 of character among themselves. 



1. Three isolated molars, belonging to the fourth and fifth of the series. They 

 are between a fourth and a third worn away, and have their masticating surfaces 

 represented in figures 13, 14, 15. They approximate the specimens above described 

 sufficiently to be viewed as pertaining to the same species, and do not vary from 

 them more than they do among themselves. 



The three specimens difier a little in proportions, degree of curvature, and com- 

 plexity of arrangement of the enamel. The smallest one, figure 15, probably a fifth 

 of the series, approaches most closely the Bijou Hill and Texas specimens in its pro- 

 portions and degree of curvature, but is more complex in the arrangement of its 

 enamel around the islets of the masticating surface, as it is also m comparison with 

 its companions. In this respect it most nearly resembles the fragment of a molar 

 among the Bijou Hill specimens. 



A second specimen, probably the fourth of the molar series, figure 14, is larger, 

 less curved, and not quite so complex in the arrangement of the enamel around the 

 central lakes of the masticating surface as in the former. The remaining specimen, 

 figure 13, hkewise a fourth molar, is much larger than the others, and the simplest 

 in the arrangement of its enamel foldings. 



