286 



ON THE EXTINCT MAMMALIA OF 



HiPPARION AFFINE. 



Among the collection of equine teeth from the Niobrara River, there are a number 

 of specimens larger than those referred to Hipparion speciosum, but having about the 

 same size and proportions as those of H. oecidentale, or of the existing Ass. They 

 however differ from those of H. oecidentale iu the simplicity of arrangement of the 

 enamel, which is not more folded than in the Horse. The internal enamel column 

 is also not only proportionately very much wider than in H. speciosum, but also abso- 

 lutely wider than in M. oecidentale. As well-marked anatomical characters appear to 

 distinguish these teeth from those of the two species of Hipparion above indicated, 

 I have referred them to another species under the name of H. affine. 



The specimens belonging to this category are as follow : 



1. Five upper molars, apparently from the same individual, consisting of the 

 second and third of the right side, and the third, fourth and fifth of the left side. A 

 series, exhibiting the triturating surfaces, from the second to the fifth inclusive, is 

 represented in figures 20 — 23. 



The teeth are much younger than those of H. oecidentale, and are comparatively 

 but little worn. They are just at that age when the third permanent molar had 

 assumed its position in the functional series, but is not sufficiently worn to exhibit 

 the entire course of the characteristic lines of enamel on the triturating surface. 



The triturating surfaces exhibit an extreme simplicity in the arrangement of the 

 enamel, compared with the condition observable in recognized species of Hipparion, 

 and is only equalled by the arrangement in the Horse and Ass. From the young 

 condition of the teeth, I at first supjDosed the simplicity was in a great measure due 

 to this circumstance, but on cutting through the second molar near its middle, I 

 found that the surfaces thus exposed still retained the simplicity of arrangement of 

 the masticating surface. 



The internal column of the crown is comparatively wide, and of uniform breadth 

 throughout its length. In transverse section on the triturating surface it is reniform. 



The measurements of the specimens are as follow : 



2d superior molar. 3d. 4th. 5th. 



Lines. 



Lines. 



Lines. 



Lines. 



Antero-posterior diameter. 

 Transverse diameter, 

 Length externally. 

 Length internally. 

 Breadth of internal column. 

 Breadth of do. at bottom, 



