1st superior 



molar. 



2d. 



6tli. 



Lines. 





Lines. 



Lines. 



. 11 





9i 



Si 



. 9 





8 



7i 



. 5* 





10* 



12 



. 3 





6i 



8 



DAKOTA AND NEBRASKA. 299 



3. The remaining specimen is a last superior molar, about a fourth worn. The 

 length of the tooth appears too great in comparison with the others referred to this 

 species, though its short diameters hold the same proportions. The triturating sur- 

 face, represented in figure 51, resembles that of the corresponding teeth referred to 

 Proio7iij)pus, to which the specimen may belong. 



The measurements of the Texas specimens are as follow : 



Antero-posterior diameter, 

 Transverse diameter, . 

 Length externally, 

 Length internally. 



Meetchippus mirabilis. 



A second species oi MerycMppus, with the above name and larger than the former, 

 was inferred to have existed, from a fragment of an upper jaw of a young animal, 

 containing the second and third temporary molars, and, concealed within the jaw, 

 their permanent successors. The specimen, from the Niobrara River, is represented 

 in figures 10 — 13, plate XVII, and is particularly described in an account of the 

 genus, in which the attempt was made to establish its generic identity with the fossil 

 from Bijou Hill, referred to Merycliippus insignis. 



Additional specimens from the Niobrara River, apparently referable to Merychippus 

 mirabilis, are as follow : 



1. Two temporary molars, the first and second of the series, represented in figure 

 14, plate A. They are more worn than the temporary teeth of the above mentioned 

 fragment, being only half their length. The second tooth agrees with that of the 

 latter in its proportions and anatomical structure. In the case of the first molar, 

 absent in the jaw fragment, there is a near resemblance in anatomical constitution 

 with the corresponding tooth of the Bijou fossil referred to M. insignis, the only differ- 

 ences observable being readily accounted for in the difference of age. The specimen 

 is provided with cementum on its inner side, but this substance is absent in the 

 central lakes, and appears to have been scant externally. In the second molar the 

 central lakes are partially occupied by cementum, and a moderate quantity exists 

 both externally and internally. In both teeth the anterior lake has an outlet into 

 the oblique valley between the middle and internal lobes. 



It would appear from the remains of Merycliippus thus far examined, that it is the 

 usual condition for the first and second temporary molars to have the anterior lake of 



