280 ON THE EXTINCT MAMMALIA OF 



HIPPAKION. 



Hipparion, or Hippotherium, is an extinct equine genus, the remains of several spe- 

 cies of which have been discovered in the middle and later tertiary deposits of Europe 

 and Asia. 



The skeleton of E.i2:>parion was constructed on the same general plan as that of the 

 Horse, or of the genus Equus. The extremities were supported on a single toe as in 

 the latter, but an additional pair of toes were provided for each foot, though they 

 were not sufficiently developed to reach the ground. 



The superior molar teeth, which are the most characteristic of these organs in 

 equine animals, in Hijjparwn present a striking difference from those of Equus, The 

 difference mainly depends on the extent of separation of the antero-internal column 

 of the crown from the antero-median column. In Equus the separation exists only at 

 the summits of the columns in the unworn teeth. As these are worn away by attri- 

 tion, the antero-internal column early appears on the masticating surface, in section 

 as an ellipse of enamel enclosing dentine, and conjoined by an isthmus with the 

 antero-median column. In Hipparion the separation of the two columns indicated 

 extends to near the bottom of the crown, so that, as the teeth are worn away, for a 

 long time there is observed on the inner side of the masticating surface an isolated 

 ring or ellipse of enamel, which does not join the antero-median column, as in Equus, 

 until the teeth are nearly worn out. Usually also the arrangement or course of the 

 enamel, as seen on the masticating surface of the superior molars, especially that 

 surrounding the median crescentoid lakes, is more complex in Hipparion than in 

 Equus. 



In the inferior molars, the prolongation in front of the anterior crescentoid column 

 is extended in a short transverse fold outwardly, the merest trace of which is to be 

 detected in the corresponding teeth of Equus towards the bottom of the crown. 



The remains of a small species of Hip>parion were discovered by Prof. F. S. Holmes 

 and Capt. A. H. Bowman, U. S. A., in a supposed post-pliocene formation on the 

 Ashley River, in the vicinity of Charleston, South Carolina. They consisted of two 

 upper molar teeth, and are described and figured, under the name oi Hipparion venus- 

 twm, in Holmes' Post-pliocene Fossils of South Carolina, page 105, figures 32, 33, 

 plate xvi. 



Since the discovery of the Hipparion venustum, numerous remains of other species 

 have been discovered in Nebraska by Dr. F. V. Hayden. As shown by the existing 

 species of Equus, we may have different extinct species represented by remains of the 

 same anatomical character, and therefore unrecognizable. The Nebraska Hipparion 

 remains, however, do exhibit differences sufficient to justify their reference to several 

 distinct species, until the discovery of more abundant material may prove otherwise. 



