DAKOTA AND NEBRASKA. 287 



2. A water-rolled specimen of a last upper molar tooth, one of the few fossils in 

 the Niobrara collection presenting that condition. It is between a third and one- 

 half worn away, and the triturating surface, as represented in figure 24, accords in 

 the simplicity of its constitution with that of the pi-eceding specimens. 



The measurements of the tooth, making some allowance for accidental attrition, 

 are as follow : 



Lines. 



15 to 19 

 Hi 



9i 



Length externally, 

 Antero-posterior diameter, 

 Transverse diameter. 

 Length of internal column. 

 Breadth of do. throughout. 



10 



4 



HiPPARION GRATUM. 



A number of comparatively small equine teeth, in the Niobrara collection, 

 apparently indicate a different species of Hipparion from those represented by the 

 specimens previously described. I am, however, not only in some doubt as to the 

 specific distinction of the teeth in question from the other Hlpparion teeth, but am in 

 an equal state of micertainty as to whether they may not belong to the same category 

 as those referred to P)-otoliippus placidus. The specimens have almost the same size, 

 usually nearly the same proportions as in the latter, and some of them in a similar 

 stage of attrition exhibit nearly as much simplicity in the arrangement of the 

 enamel. They further approach in character in the extent to which the posterior 

 enamel inflection reaches in the length of the crown. In the teeth of Hlpparion 

 previously described, the inflection just mentioned nearly reaches the bottom of the 

 crown, as in the upper molars of the Horse and Ass. 



An apparently important difierence between the teeth under consideration and 

 those referred to Protohippiis placidm is in the extent of separation of the internal 

 column of the crown from the antero-median column. In the attrition to which the 

 teeth were subjected in mastication, in Protoliippus, as in the Horse, the two columns 

 just mentioned became early associated by an isthmus on the triturating surface. 



In the specimens under examination, the columns mentioned are separated a con- 

 siderable depth of the crown, as in the recognized species of Hlpparion, and they 

 appear not to have become conjoined on the triturating surface until the teeth were 

 much worn away, in which condition they are hardly distinguishable from those 

 referred to Pivtohippus placidus. 



The specimens in question are as follow : 



1. Three molars of such an age, proportions, and general appearance as to render 



