DAKOTA AND NEBRASKA. 279 



Lines. 



Length externally, . . . . . . . .8 



Length internally, . . . . . . . .5 



Antero-posterior diameter, . . . . . . .8 



Transverse diameter, . . . . . . .7 



Three additional specimens may be described under the present head, though I am 

 doubtful as to the propriety of their association, nor can I with less uncertainty refer 

 them to Hipparion gratum, hereafter to be described. They are as follow : 



A first superior molar, four-fifths worn away, and represented in figure 41. Its 

 proportions are greater than the corresponding tooth above described would have been 

 at the same stage of attrition. The central lakes of the triturating surface are of a 

 different shape, less gaping, and present, even in their much worn condition, evidences 

 of having been more complex in the arrangement of their surrounding enamel. 

 Remains of a posterior valley of the crown exist as a small reniform islet in the 

 postero-internal corner of the masticating surface. The internal column is nearly 

 circular in transverse section, and is associated with the antero-median column by a 

 wide isthmus, 



A first superior molar, of greater proportions, but more worn than the preceding 

 specimen. It is represented in figure 42, and more nearly resembles the latter than 

 it does the first-described tooth referred to P. placidus. On the triturating surface all 

 traces of the distinction of internal columns have disappeared, so that from the 

 specimen alone it would be very uncertain whether to refer it to PivtoJiippus or 

 Hijyparion. 



A second or third superior molar, four-fifths worn away, and represented in figure 

 48. It accords in proportions and size with the teeth above referred to P. placidus 

 generally. The central lakes of the triturating surface are still capacious, and in the 

 specimen are both nearly devoid of cementum. The remains of a posterior valley 

 exist as a small circular ring in the back internal corner of the masticating surface. 

 The internal column is associated with the antero-median column by a very narrow 

 isthmus. The tooth I have suspected to belong to a Hipparion, so much worn away 

 that, having nearly reached the bottom of the crown, the two columns, usually 

 observed separated, have become conjoined. 



The measurements of the three specimens are as follow : 



Length externally, . 

 Length internally, 

 Antero-posterior diameter, 

 Transverse diameter. 



t superior molar 



1st. 



2d. 



Lines. 



Lines. 



Lines. 



. 31 to 6 



3 to4J 



4 



. 2i 



li 



3 



. 10 



11 



9 



. 8 



8 



8 



