151 



[Cope. 



traction of its connection with the corresponding crescent. The section 

 of this loop is a rather wide oval. The posterior loop has half the size, 

 and if isolated would present the same form. 



The crown of the tooth is of median length and is strongly curved in- 

 wards. Its grinding surface is a little wider than long, and is worn into 

 two transverse angles, which pass through the concavities of the borders 

 of the crown and lakes. It is not certain that this grooving in wear is a 

 constant character. 



The lakes are strongly convex inwards and their horns are wide and 

 obtuse. Their borders are simple, there being no folds on the remote 

 sides, and on the adjacent borders only one on the posterior and two on 

 the anterior, of no great depth. There is no loop at the junction of the 

 inner edges of the internal crescents. External ribs of crown prominent. 

 Excepting these, the entire crown is enclosed in cementum. 



Measurements. M. 



Length of crown 040 



Diameters of grinding face { anteroposterior 021 



C transverse 023 



This species differs from the P. insignis, P. perditus and P. mirabilis, 

 with which it agrees in size, in the posterior production and angulation of 

 the posterior border of the anterior inner column, and in the absence of 

 plication of the borders of the lakes which are remote from each other. 

 In this species the internal loops are of nearly equal size. I have dedi- 

 cated it to my distinguished friend, Prof. Antonio de Castillo, Director of 

 the School of Mines of the City of Mexico, to whom I am indebted for a 

 knowledge of the locality described in the present paper. 



Fig. 1. Fig. 2. 



Fig. 1. Superior molar tooth of right side of Hippotherium peninsulatum Cope 

 grinding surface from below ; natural size. 

 Fig. 2. The same of Prolohippus caslilli Cope ; same view ; natural size. 



