the H. occidentale, H. speciosum, and H. gratum of Leidy, in the 

 relative form and size of the interior dentinal column. In the two 

 species first named, this column is subcylindric and equal to or 

 smaller than the posterior internal columnar fold; in the three 

 species last named the anterior column is flattened or oval in 

 section, and often larger than the posterior columnar fold, and 

 submedian in position. 



In the typical or New Mexican specimen the anterior column is 

 large, and its centre is anterior to the middle transverse line of the 

 crown. In the present state of attrition, which has left two-thirds 

 of the crowns of the median molars, this column presents an 

 angular projection towards the inner anterior crescent, betraying 

 an approach to the union seen in Protohipjms, which is in the 

 fifth molar of the right side of the present horse, actually accomr 

 plished through the medium of a narrow isthmus. The dentinal 

 band, connecting the inner crescents, throws out two, rarely one, 

 fold towards this column. The borders of the lakes are much 

 plicate, the posterior border of the anterior lake having from 

 four to six inflections. The posterior border of the posterior lake 

 exhibits one deep inflection, which is generally bifurcate. 



The first premolar is quite small, and is two-rooted ; the second 

 is a wide tooth, different in form from the elongate corresponding 

 tooth of the H. occidentale, its anterior lobe being but little 

 prominent. The palate is wide and well arched. The mandibular 

 teeth are elongate, but not narrow, and the interior lobes are well 

 developed, especially the median ones. No basal cingula on teeth 

 of either jaw. The last molar is smaller than the three preceding 

 it, and the anterior lobe of the first is quite narrow. 



The molars of one of the specimens from Colorado are closely 

 similar in all respects, except that the anterior lobe of the second 

 premolar is a little more produced. I also refer here the tooth 

 described as " No. 2," under H. paniense, in Report U. S. Geol. 

 Survey, Terrs. 1873, p. 522-3. As compared with the molars of 

 that species those of H. calamarium differ in the much greater 

 complexity of the enamel folds, those of the H. paniense being the 

 simplest in the genus, even more so than in H. affine. The ante- 

 rior inner column of H. paniense lacks the characteristic inner 

 angle seen in the present horse. 



Measurements. 

 Length, of seven superior molars . 

 " " three true molars . 

 " " second premolar 

 Width of crown of second premolar 

 Greatest length of second premolar 

 " width " " " 



" length " penultimate molars 

 " width " " " 



Width of palate (chord) between middle of third pre 



molars 



Length crown antepenultimate lower molar . 

 Width " " " " 



M. 



.134 

 .062 

 .026 

 .020 

 .022 

 .024 

 .022 

 .022 



.042 

 .021 

 .012 



