DAKOTA AND NEBRASKA. 277 



P. perditus. E. asinus. E. caballus. 



Lines. Lines. Lines. 



Distance from alveolar edge to infra-orbital foramen 



above third molar, .... 18 23 33 



Distance from infra-orbital foramen to anterior orbital 



margin, ...... 2G 38 54 



Among the multitude of equine remains in the Niobrara and other collections, there 

 are no others which can be referred with any certainty to P)-otohij)]ius perditibs. 



Protohippus placidus. 



Among many isolated upper molars of equine animals from the Niobrara Eiver, 

 mostly of uncertain reference, there are several sufficiently resembling those of Proto- 

 hippus perditus to lead to the suspicion that they may belong to another and smaller 

 species of the same genus. The teeth alluded to are not only smaller, but are less 

 curved in relation with their length than in P. perditus. With the impression that 

 they represent a distinct species, they may be referred to under the name o£ Proto- 

 hippus placidus. The specimens apparently referable to it are as follow : 



1. A first upper molar tooth about half worn away. The masticating surface, 

 represented in figure 40, plate XVIII, presents extreme simplicity in the arrange- 

 ment of the enamel, compared with its condition generally in equine animals. The 

 central lakes appear wide and gaping, as in the more posterior teeth of Protohipjms 

 perditus. No trace of a posterior valley or enamel inflection of the crown exists. In 

 this character the tooth approaches those of P. perditus, for although the inflection is 

 present in the molars belonging to the fossil of the latter, it evidently extends but 

 little depth in the length of the crown. In the fourth molar of the fossil just 

 mentioned the inflection has been nearly obliterated, its remains being visible as a 

 small ring in the postero-internal corner of the triturating surface ; and at a little 

 later period in the progress of attrition, the inflection would likewise have dis- 

 appeared. The measurements of the specimen are as follow : 



Lines. 



Length externally of the first upper molar, . . . . 9 to 10 



Length along the internal column, . . . . . .7 



Antero-posterior diameter of triturating surface, . . . .91 



Transverse " " " • • • . 7J 



2. Two upper molars, seconds or thirds, or one of each of the series, very much 

 worn. One of the specimens, the triturating surface of which is represented in figure 

 47, is about half worn away. It exhibits the same simplicity of arrangement of the 

 enamel as in the preceding specimen and in the teeth of P. perditus. The other spe- 



