DAKOTA AND NEBRASKA. 



289 



Lines. 



Length of crown along internal column, ..... 17i j 

 Breadth of internal column at triturating surface, . . . .4 



Breadth of internal column at middle of crown, . . . . 3J 



3. A fourth or fifth molar, from a third to one-half worn, represented in figure 30, 

 plate XVIII. It accords in character with the less complete fifth molar of the fore- 

 going series, described previously to the last specimen. Its measurements are as 

 follow : 



Lines. 



Antero-posterior diameter, . . . . . . .8 



Transverse diameter, . . . . . . .8 



Length externally, . . . . . . . .15 



Length internally, . , . . . . . .9 



Breadth of internal column, . . . . . .3 



4. Two last molars but little worn, represented in figures 28, 29, plate XVIII, 

 The measurements of the specimens are as follow : 



Shorter one. 

 Lines. 



Antero-posterior diameter just above the rounded triturating 



extremity, ..... 



Transverse do. including cementura, . 

 Antero-posterior diameter at bottom of crown, 

 Transverse " " " . 



Breadth of internal column, .... 

 Length externally, ..... 

 Length internally, ..... 



Eipparion-like ttpper molar teeth of uncertain reference. 



The Niobrara collection contains a number of Hipparion-\ikQ molar teeth, both 

 permanent and temporary, of the upper jaw, which I have been unable to refer to the 

 foregoing species with any degree of certainty, nor do I feel more satisfied that they 

 belong to other species. The specimens are as follow : 



1. Three isolated upper molars, belonging to the second and third of the series. 

 They are from about a fourth to a third worn away, and are represented in figures 

 31, 32, 33, plate XVIII. Their proportions are somewhat less than the White River 

 specimens of Hijyparion occidentale, but their triturating surface exhibits as much 

 complexity in the arrangement of the enamel. Their internal column is of variable 

 diameter and shape. In one it is like that most usual in the teeth referred to H. spe- 

 ciosum; in another it is as broad as in H. occidentale; and in the third specimen it is 



of intermediate size. 



37 



8 



7 



9 



8 



2} 

 17 

 14 



Longer one. 



Lines. 



7J 



6 



8i 



7 



3J 

 18J 

 15i 



