16 



a portion of the left mandibular ramus, in which only the last molar is 

 sufficiently well preserved for identification. The latter is, however, per- 

 fect, and furnishes clear evidence of the former existence on the west side 

 of the Rocky Mountains of a species distinct from the H. americanus Leidy 

 from the more eastern regions. The cones are in pairs and are directly 

 opposed ; their section is sub-trihedral, the two external sides of the ex- 

 ternal cones, forming a regular convexity. The cusps are acutely pro- 

 duced and slightly divergent. The posterior side of each outer cusp is 

 excavated ; the exterior side of the same presents a median rib with a con 

 cavity On each side, which is terminated below by an imperfect cingulum. 

 The latter terminates on each side of the base of the cusp in a rudimental 

 cusp, of which there are thus four on the external side of the tooth. The 

 boundaries of the inner face of the external cusps are angular ; the poste- 

 rior one joins a corresponding ridge from the inner cusps, but there is no 

 descending ridge on the anterior inner side of the internal cusp, which 

 therefore forms no junction with the opposite part of the external cusp. 

 The fifth cusp is well developed, and sends a crest inwards to the interior 

 base of the interior cusp of the adjacent pair. 



Measurements. M. 



r.. , P , , . f . , f antero-posterior 022 



Diameter of last interior molar { l 



t transverse 010 



This species is smaller than the H. americanus, and differs much in de- 

 tails. It is dedicated to Prof. Arnold Guyot, of Princeton, New Jersey. 



Printed Dec. 8, 1878. 



