DAKOTA AND NEBRASKA. 



327 



Length at the middle externally, 



Breadth of triturating surface, . 



Breadth below, ..... 



Width of triturating surface, 



Breadth of internal column, 



4. Portions of a number of upper molars, together with a perfect one, having the 

 general aspect as if they had belonged to the same animal. A full series is repre- 

 sented in figure 1, plate XXVII, in a restored condition. They bear a strong likeness 

 to those 0^ ProtoMppus perditus, represented in figure 2, plate XVII, and may belong 

 to that species. The teeth are, however, larger than those in the specimen re- 

 ferred to P. perdihis, and have their internal columns more cylindrical, or circular 

 in transverse section, as is the case in MerycMppns. It is probable they may belong 

 to this genus, which, in the upper molar teeth, is apparently only distinguishable from 

 Proiohippus in having the internal columns of the crown for the most part regularly 

 cylindrical instead of compressed cylindroidal. The apparent greater difference 

 observed in the interior lakes of the triturating surfixces in figures 5 and 9, plate 

 XVII, oi Merycldppiis, and figure 2 oi Protohippxis, depends simply on a difference in 

 the degree of abrasion of the teeth. In the comparatively little worn specimens of 

 F^otohippus, figure 2, the lakes appear wide and gaping. As these lakes contract 

 towards the bottom of the crown, when this is much worn they would appear nar- 

 row, as in the specimens of Merycliippus, figures 5 and 9. 



The teeth under consideration approach in size and proportions those of Merycliip- 

 pus mirabilis, and probably belong to this species. The measurements derived from 

 the impei'fect specimens are as follow : 



Lines. 



Length of first molar externally at middle, . . . . . 14i 



Breadth of do., ........ 13* 



Width from external median ridge to internal median column, . . 10 



Breadth of second molar (partially estimated,) . . . .12 



Breadth of third molar, an .... 12i 



Length of fourth molar externally, . . . . . .18 



Breadth of fourth molar, " . . . . . .11 



Breadth of fifth molar, " . . . . . -Hi 



Breadth of sixth molar, " . . . . • .11 



5. An isolated molar, apparently the fourth of a series, represented in figure 5, 

 plate XXVII. It bears a near resemblance to the corresponding tooth oi Protohippus 

 perditus, in the specimen represented in figure 2, plate XVII, and probably belongs to 

 the same species. Its measurements are as follow : 



