VERTEBEATE FAUNA OF THE LOUP FORK BEDS. 43 



We learn then, that the emargination of the enamel borders of the lakes 

 are not characters of immaturity. As compared with the Pampeian 

 species,* the enamel borders of the H. interpolatum are more simple; 

 while they are not so simple as in the species from Oregon which I have 

 called H. spedans.'f In the latter the posterior lake borders are not 

 emarginate, and their external borders are more deeply concave. The 

 lakes are wider, a character which may be due to wear; but a further 

 difference is the greater posterior production of the protocone, which 

 considerably overlaps the line of the anterior border of the posterior lake. 

 The H. interpolatum is of about the size of the H. spectans and H. neogeum. 



HIPPIDIUM FSPECTANS, Cope. 



American Naturalist, 1887, fig. 41. Huj. op. Plate XII, Figures 5, 6. 



Two superior molars belong to a species allied to if not identical with 

 the one above named. One of these is considerably worn, while the 

 other, the posterior of the left side, is little worn. Both display charac- 

 ters different from those of the H. interpolatum, as follows: The proto- 

 cone is large, and its posterior border is opposite the middle of the pos- 

 terior lake. The enamel borders of the lake are simple, a slight notch in 

 one lake of the younger tooth excepted. The size is smaller. The di- 

 mensions are about equal to those of the ProtoJiippus supremus of Leidy, 

 but in that species the anterior lake has a large loop, and the lake borders 

 are elsewhere folded. It agrees with the H. spectans, especially in the 

 simplicity of the lake borders, which the young molar shows is not a 

 character of age, as in most other species of horses. The hypocone is not 

 conspicuous in either of the teeth. Both exhibit transverse curvature, 

 and the last molar a little anterioposterior curvature. 



Measurements. 



Mm. 



Diameters of older molar \ anteroposterior 24 



( transverse 27 



i longitudinal 64 



anteroposterior 24 



transverse 20 



EQUUS EURYSTTLUS, CopC. 



Plate XII, Figures 7, 8; Plate XX, Figure 6. 



This small but remarkable true horse was first brought to my notice by 

 a broken inferior molar from the Paloduro canyon, where it was found 

 by Mr. Cummins. The horizon of this locality was unknown to me, but 



* Burmeister, Los Caballos Fossiles de la Pampa Argentina, Buenos Ayres, 

 1875. 

 t American Naturalist, 1887, p. 1072. 



