294 ON THE EXTINCT MAMMALIA OF 



Ilayden's rich collection from the Niobrara River was submitted to my examination. 

 In the collection there are a number of fossils which appear to me to belong to Mery- 

 chipjms, though I am not positive of the correctness of my view, but feel that the 

 matter must be left to more acute powers of observation, or to the discovery of new 

 material, to determine the question. 



One of the fossils of the Niobrara collection, represented in figure 10, plate XVII, 

 and apparently referable to the same genus as the Bijou fossil above described, con- 

 sists of a portion of the right side of the upper jaw of a young equine animal, contain- 

 ing in functional position the second and third temporary molars, and within the jaw 

 the corresponding teeth of the permanent series. 



The triturating surfaces of the temporary teeth, represented in figure 11, plate 

 XVII, are much more worn than in the Bijou fossil, but on comparing the correspond- 

 ing tooth — that is to say, the second molar — in both specimens, it will be observed 

 that in a more worn condition the tooth of the Bijou fossil would have assumed a 

 strong likeness to that of the Niobrara fossil. The difference between the two 

 appears greater from a difference in size, and from the presence of cementum in the 

 Niobrara fossil and its absence in the other. The similarity in the two teeth indi- 

 cated appeared to me to be sufficiently great to I'efer the fossils to the same genus; 

 the difference in size, apparently confirmed by other specimens, has led me to refer 

 them to different species. Notwithstanding this determination, I feel px'epared to find 

 through more accurate observation or further discovery that my view is erroneous. 



For the smaller species, first indicated by the Bijou fossil, the name oi Merychippiis 

 insignis has been proposed ; for the larger one, to which the Niobrara fossil belongs, 

 that of Merychip'pvs mirabiHs. 



The temporary molars in the latter fossil, as before intimated, are provided with 

 cementum, from which circumstance they present a more striking resemblance to the 

 corresponding teeth of the Horse than those of the Bijou fossil. In the second molar, 

 however, the cementum appears to have been somewhat scanty, as it only partially 

 fills the central lakes of the crown, adhering to their parietes. The third temporary 

 molar, from the presence of the cementum and the greater proportionate length of the 

 crown, bears less resemblance to the molars of Anchitherium than the tooth in 

 advance. 



Both temporary teeth of the Niobrara fossil are inserted by fangs, and the crowns 

 of both are much more worn than in the Bijou fossil. The worn triturating surfaces 

 bear a near resemblance to those of the Horse in the same condition. The course of 

 the enamel surrounding the central lakes, as also that at the periphery of the crown, 

 is as simple as in the Horse. 



The anterior lake of the triturating surface of the second molar, in the Niobrara 

 fossil, has an outlet into the oblique valley between the median and internal lobes, as 



