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 HZ. americanus Leidy 
from the more eustern 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 fice 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. 
f antero-posterior....... .022 
tramsverse..........- 010 
Diameter of last inferior molar 
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. 3, 1878. 
