DAKOTA AND NEBRASKA. 



275 



Height of anterior slope, 



Breadth of lateral spread, 



Extent of spread antero-posteriorly, 



Breadth of articular surface, 



Projection of angles laterally beyond the latter, 



Lines. 



23 

 26 

 24 

 24 

 3 



PROTOHIPPUS. 



In this genus, as in Equus, the superior molar teeth have their antero-internal 

 column associated with the antero-median column throughout the length of the crown. 

 It is, however, very little larger than the postero-internal column, — extending only 

 backward of its connection with the antero-median column, as in the first upper per- 

 manent molar of Equus and as in the temporary series of this- genus. The corres- 

 pondence in the character mentioned between the upper permanent molars of 

 Protohijypus and the corresponding first tooth, together with the temporary molars of 

 Equus, indicates an earlier or more primitive condition of the former genus. The 

 arrangement of the enamel in ProtoMppus is even less complex than usual in Equus. 



Protohippus perditus. 



Dr. Hayden's Niobrara collection of fossils contains an interesting specimen, con- 

 sisting of a fragment of the skull of an equine animal, smaller than the Ass, to which 

 the above name has been given. The fragment is composed of the back portion of the 

 right maxillary bone, with the greater portion of the contiguous malar and a small 

 portion of the lachrymal bone, and it contains the back four molar teeth. The infra- 

 orbital margin and part of the infra-orbital foramen are preserved. 



The specimen, represented in figure 1, plate XVII, closely resembles in form and 

 construction the corresponding part in the Domestic Horse. As in this, the malar 

 ridge of the maxillary terminates above the position of the antepenultimate molar, 

 and the infra-orbital foramen is situated above the tooth in advance. The only 

 important difference observable in the fossil as compared with the skull of the Horse, 

 is in having the orbit a little more anterior in position, as in the Ass, — its front border 

 being on a line with the fore part of the last molar tooth. 



The teeth, consisting of the back four molars, are but moderately worn, and are 

 more curved in relation with their length than in the Horse. The antei'ior tooth, 

 exposed to view at the fore part, presents an external curvature the radius of which 

 is about twenty lines; the internal curvature is the segment of a relatively smaller 

 circle. Its crown measures eighteen lines along the former curvature and nine lines 

 along the latter. 



