DAKOTA AND NEBRASKA. 293 



proportions, and relation with each other as in Anchitherium aurelianense, with which 

 they also nearly agree in size. The internal lobes, compared with their condition in 

 the latter animal, are much reduced in size, while the median lobes are enlarged in a 

 proportionate degree. In Aiicliitlierium the external and internal lobes are the 

 principal ones, while the median lobes appear to be of secondary importance. In 

 Merychippus the external and median lobes, like the corresponding columns in the 

 upper molars of the Horse, are the principal ones, and the internal lobes are of 

 secondary importance, likewise as in the Horse. 



In Ancliitherium the median lobes of the crown of the upper molars curve inward 

 and backward, and abruptly cease by becoming continuous with the internal lobes. 

 They appear to be arrested in their course or backward sweep by a comparatively 

 excessive development of the internal lobes. In Merychippus the median lobes are 

 crescentoid demicones like the internal ones in the true molars of the Deer, and they 

 hold the same relation to the external lobes as the latter do in the Deer. A compara- 

 tive reduction in development of the internal lobes has allowed a full sweep of the 

 median lobes, so as to include in their embrace the external lobes. The internal 

 lobes of the crown are simple cones springing from the inner side and base posteriorly 

 of the median lobes. 



The antero-median lobe in transverse section in the second molar forms a simple 

 crescent. In the first molar its anterior horn is complicated with several minute 

 folds or processes. The postero-median lobes in both teeth have their horns compli- 

 cated by conspicuous offsets or diverging processes. 



At the back part of the crown of the upper molars of AncJiitherium there is a con- 

 spicuous tubercle, apparently an offset from the basal ridge. In Merycldppits the 

 corresponding process is associated as a constituent portion of the postero-median 

 lobe, and appears as its complex posterior horn. 



The processes complicating the horns of the median lobes of the crown, in the teeth 

 of Merycliippjiis, evidently correspond with the folds observed in a similar position 

 upon the median columns of the upper molars in the Horse, which become conspicu- 

 ous as tortuous enamel lines bounding the sides of the central lakes of the worn 

 triturating surfaces in that animal. 



The measurements of the teeth in the Bijou fossil are as follow : 



1st superior molar. 2d. 

 Lines. Lines. 



Length externally at the middle of the lobes, 

 Length externally at the intermediate ridge, 

 Antero-posterior diameter, .... 



Transverse diameter, . . , . . 



Fifteen montlis subsequently to the notice of the fossil above described, Dr. 



4J 



5J 



4 



4J 



1 



9* 



8i 



9 



I 



