282 ON THE EXTINCT MAMMALIA OF 



Antero-posterior diameter of triturating surface, . 

 Transverse do. (indej^endent of the cement) , 

 Length of croAvn externally, 



Breadth of internal column at the triturating sur- 

 face, from before backward, . 

 Breadth of do. at bottom of the crown, 



HiPPARION SPECIOSUM. 



Shortly after the discovery of the teeth of Eipparion occidentale on White Kiver, 

 Dr. Hayden obtained several specimens of equine molars at Bijou Hill, situated east 

 of the Missouri River, below the outlet of the White River. In a notice of these 

 fossils in the Pi'oceedings of the Academy for 1856, page 311, they were referred to a 

 species under the narae o{ Hipparioji sjieciosum. Several of the sjjecimens ascribed 

 to the latter ajjparently belong to a distinct equine genus, Merychippus, hereafter to 

 be described. The others consist of an unworn upper molar, a second partially worn, 

 and two imperfect specimens. 



The unworn molar, a fourth or fifth of the series, is smaller and more curved than 

 the corresponding teeth of H. occidentale. It also decreases in diameter towards the 

 bottom of the crown more rapidly than in the latter, and the internal column is of 

 less uniform diameter. 



The second specimen, belonging to the same position in the series as the former 

 one, resembles it in size, proportions and curvature. It is about a fourth worn, and 

 exhibits the characteristic relation of the constituent elements on the masticating 

 surface, as represented in figure 16, plate XVIII. The arrangement of the enamel 

 around the median crescentoid lakes is not more complex than in the Ass, and there- 

 fore less than has been described to be the case in other known species of the genus 

 Hippiarion. The internal column of the crown is relatively narrower than in //. occi- 

 dentale, and in section is oval. The measurements of the sj)ecimen are as follow : 



Lines. 



Antero-posterior diameter, . . . . . . .9 



Transverse diametex', . . . . . . .81 



Length externally, . . . . . . . .17 



Length internally, . . . . . . . .10 



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



The imperfect specimens likewise belong to the fourth and fifth of the upper molar 

 series, and accord in their construction, size, proportions and curvature, with the 

 preceding. Their triturating surface, represented in figures 18, 19, exhibits nearly 



