DAKOTA AND NEBRASKA. 313 



lobe with the appearance of having a disposition to conjoin it, and thus form together 

 a crescentoid lobe embracing the antero-external lobe, as in the corresponding 

 columns of the Eqnidce. No similar arrangement exists in the true Anchitherium. A 

 crescentoid tubercle, as in the latter genus, occupying the interval at the back of the 

 crown, gives off a process, which joins the contiguous internal lobe near the starting 

 outwardly of the postero-median lobe. In other words, the crescentoid tubercle ex- 

 hibits a disposition to join the latter, thus together to form a crescentoid lobe, em- 

 bracing the postero-external lobe as in the corresponding columns of the Equidoe. 

 Constituent portions of a basal ridge exist anteriorly and posteriorly as in Anchithe- 

 rium. 



In the specimen, the summits of all the lobes are worn so as to exhibit tracts of 

 dentine. The tracts upon the summit of the internal lobes form ellipses with their 

 long diameter oblique, and in the direction of the median lobes. In Anchitherium the 

 corresponding exposed tract on the summit of the antero-internal lobe has its longer 

 diameter transverse. 



The tooth, from its structure, is evidently intermediate to those oi Merychippus as 

 a representative of the Equidce, and those of Anchitherium. 



In its perfect condition the tooth has been about ten lines in its antero-posterior 

 diameter, and eleven in its transverse diameter. 



PARAHIPPUS. 



In this genus, as defined from three upper molar teeth and a single inferior molar, 

 apparently of the temporary series, the following peculiarities are observed : The 

 external lobes of the crown of the upper molars on their outer surface swell into a 

 strongly marked median ridge. The median lobes are as well developed in proportion 

 with the inner lobes as in the preceding genus. The postero-median lobe divides at 

 its outer extremity into two widely diverging processes, each of which sub-divides. 

 The teeth show a nearer approach to those of the Equidce, through the genus Mery- 

 chippus, than in any of the preceding genera. In other words, there is not only an 

 increase in the size of the median lobes and a corresjwnding reduction of the internal 

 ones, as compared with Anchitherium and Rypohifpus, but there is exhibited a dispo- 

 sition to union of the median lobes and posterior tubercle of the crown, as in Anchip- 

 pus, and in addition we have the anterior or outer extremity of the postero-median 

 lobe complicated with additional processes as in Merychipptcs. In the inferior molar, 

 the internal conical lobes as seen in Anchitherium are developed into twin cones, 

 corresponding with the internal twin columns of the lower molar teeth of the Horse. 



40 



