DAKOTA AND NEBRASKA. 311 



IIYPOIIIPPUS. 



Regarding the characters of this genus as distinct from the former, so far as they 

 are derived from a single upper molar tooth, they are as follow : The external lobes 

 have the same form as in Anchitherium, but in the specimen their outer face is trans- 

 versely concave, without a trace of median ridge. The internal lobes form regular 

 cones, the transverse and antero-posterior diameters being equal. 



Hypohippus affinis. 



The above name has been applied to a genus and species of a supposed solipedal 

 animal, allied to Anchitherium, and inferred to have existed from a single fossil tooth, 

 in the Niobrara collection. The specimen consists of a well-preserved crown of an 

 upper molar, and is represented in figures 11, 12, plate XXI. From its hoUowness, 

 together with its worn condition, I suspect the tooth to be the second or third of the 

 temporary series. 



The crown has the same anatomical construction as in Anchitherium, being com- 

 posed of three pairs of lobes having the same relative position with one another, and 

 about the same proportions as in that genus. 



The tooth is larger than those oi Anchitherium aurelianense, and is equal in size to 

 those of Palceotherium medium, with which it also bears a general resemblance. It 

 has been inserted into the jaw by fangs, as in the two latter animals, and appears to 

 have been equally devoid of cementum. It is worn so as to exhibit wide tracts of 

 dentine along the summits of its constituent lobes. 



The external lobes of the crown have the same form as in Anchitherium, and are 

 sustained by equally robust buttress-like ridges, but their outer face is uniformly con- 

 cave, and not interrupted by a median ridge. The internal lobes are regularly coni- 

 cal, the diameter from before backward being equal to the transverse, whereas in 

 Anchitherium the latter exceeds the former, thus giving the internal lobes in this 

 genus the appearance of compressed cones. The antero-internal lobe is considerably 

 larger than the one behind. The median lobes are even less dilated at their inner 

 part than in Anchitherium. The outer extremity of the postero-mediau lobe is con- 

 tinuous with the conjunction of the external lobes, and just before its termination, in 

 front and behind, exhibits a small process or tubercle, apparently the rudiment of the 

 complex folds, in a corresponding position of the teeth of the Equidce. A crescentoid 

 tubercle occupies the interval at the back of the crown, and constituent portions of a 

 basal ridge are situated around the tooth exactly as in Anchitherium. 



The measurements of the specimen are as follow : 



