DAKOTA AND NEBRASKA. 295 



in the first and second molars of the Bijou fossil, but both lakes are closed in the 

 third molar. 



The antero-internal lobes are conoid, and their worn summits exhibit a nearly 

 circular section, continuous with the antero-median lobe, as in ProtoMppus, and as is 

 also the case in the temporary molars of the Horse. 



The permanent molars preserved in the interior of the Niobrara jaw fragment con- 

 sist of the second, and of the third imperfect at its posterior part. The jaw fragment 

 also retains part of the cavities for the first and fourth molars. The latter tooth had 

 protruded, and appears to have been about the size of the teeth of the Ass, or those 

 in the preceding pages referred to Hipparion affine or H. occidentale. 



The permanent teeth preserved in the fossil, as represented by the second one in 

 figures 12, 13, plate XVII, are in the condition of thin dentinal shells, thinly in- 

 vested with fissile enamel and devoid of cementum. They are constructed on the 

 same plan as the teeth of the Horse, resembling them in the same condition of de- 

 velopment, except that they present the character assigned to P)-otohippus, — namely, 

 the antero-internal column is narrow, and does not expand in advance of its con- 

 junction with the antero-median column, which, as repeatedly' stated, is likewise 

 the condition in the temporary molars of the Horse. 



Had these teeth been observed as isolated specimens in a more advanced stage of 

 development, mingled with the other equine teeth of the Niobrara collection, they 

 would have been referred to Pi'otoliippus, but important differences in the character 

 of the jaw fragment in which they belong establish their distinction. This circum- 

 stance appears less remarkable from the fact that we have a number of distinct 

 species of Equus, without correspondent differences in the teeth or parts of the 

 skeleton. 



The portion of the upper jaw of ProtoMppus perditus, figure 1, plate XVII, pre- 

 viously described, was stated to be the counterpart in form of the corresponding por- 

 tion of the face in the Horse. 



In the Niobrara fossil referred to Merychippus viirabilis, fortunately for comparison, 

 nearly the corresponding portion of the jaw has been preserved, figure 10, as in Pro- 

 tohip>pus perditus. The malar ridge of the maxillary bone ceases above the position 

 of the last temporary molar tooth. Immediately above the ridge, including its upper 

 surface, the bone is impressed with a remarkable fossa, broad and deep, and recalling 

 to mind the lachrymal depression of the Deer. . A similar ant-orbital depression, 

 though situated higher in its relation with the malar ridge, existed in the more nearly 

 related Hipparion vied iter raneum, as seen in a well-preserved skull, from Greece, de- 

 scribed and represented by Dr. A. Wagner in the fifth volume of the Transactions of 

 the Royal Bavarian Academy of Sciences, page 338, plate ix. 



The ant-orbital maxillary depression in the fossil under examination is encroached 



