DAKOTA AND NEBRASKA. 145 



their width at base appears somewhat greater. Their outer surfaces are more oblique 

 in relation with one another than in the Camel. 



As in the latter, the outer surfaces of the external lobes of the upper true molars 

 are nearly vertical planes, with narrow median ridges and bounded anteriorly by 

 prominent narrow ridges. The inner surfaces slope moderately and become rather 

 abruptly prominent along the middle. The outer surfaces of the internal lobes are 

 strongly concave ; the inner surfaces prominent, strongly sloping, and feebly concave 

 in their length. 



Of the worn crescentic summits of the inner lobes of the upper true molars, the 

 anterior horn of the posterior crescent is continuous with the conjoined horns of the 

 external crescents ; and the posterior horn of the anterior crescent is directed towards 

 the postero-internal face of the antero-external lobe. 



The second lower premolar has a crown nearly three times the breadth of the 

 length and thickness. It is feebly trilobate externally, and has a trenchant border 

 rising towards the middle. Its posterior thicker extremity extends a short distance 

 external to the fore part of the succeeding tooth. 



The third lower premolar has the crown more than three times the breadth of the 

 length. It is feebly trilobate externally, but more decidedly so internally from the 

 presence of a notch separating the anterior two divisions. It widens moderately from 

 before backward. The triturating border presents a zig-zag tract of dentine, 

 widening posteriorly. 



The lower temporary true molar is so worn that the crown appears composed of 

 three simple prismatic columns of dentine, enveloped in enamel, successively 

 enlarging posteriorly. 



The lower permanent true molars approach in form those of the Camel. Their 

 inner surfiices are quite as simple, forming nearly vertical planes, with slight median 

 and marginal elevation of the constituent lobes. The outer surfaces exhibit more 

 angular prominences of the external lobes. The outer surfaces of the internal lobes 

 are remarkable for being almost flat. The inner surfaces of the external lobes 

 are angularly concave. 



The worn triturating surfaces of the true molars exhibit narrow tracts of dentine, 

 continuous everywhere upon the constituent lobes, and enclosing wide, three-sided 

 enamel pits. 



The last molar has a fifth lobe, which is comparatively thin and has an acute 

 biting border, unworn in the fossil. 



The species Pcehrotlierium Wllsoni was named in honor of Dr. Thomas B, Wilson, 

 of Philadelphia, a distinguished patron of natural history. 



Measurements derived from the fossil are as follow : 



19 



