DAKOTA AND NEBRASKA. 329 



Lines. 



Space occupied by the temporary molars, . . . . .40 



Breadth of first molar, . . . . . . , 13i 



Breadth of second molar, . . . . . . . 12i 



Breadth of third molar, . . . . . . .14 



Depth of jaw at back of last molar, . . . . . .23 



Depth of jaw at front of first molar, . . . . .16 



Depth of jaw on a line with the mental foramen, .... 12* 



9. A fragment of a lower jaw of the left side of an adult animal, probably belong- 

 ing to the same species as the last specimen. It contains the posterior two molars. 

 These teeth, though worn down the greater part of their extent, exhibit no ti'ansverse 

 fold projecting outwardly at the fore part of the crown, as is usually the case in 

 Eipparion. 



Probably this and the preceding specimen belong to Protohippus. 

 The space occupied by the two teeth is an inch and a half; the breadth of the fifth 

 molar is eight and a quarter lines; of the sixth molar, ten lines. 



10. A fragment of the right side of a lower jaw, containing the posterior three 

 molars of the permanent series. It belonged to a rather larger animal than the pre- 

 ceding specimen, and one of about the same age, but evidently to a difierent species, 

 as indicated by the difference in arrangement of the enamel on the worn triturating 

 surfoces. The teeth of this fossil are of the character of those of Eipparion, as indi- 

 cated by the presence of the antero-external fold of the crown. The measurements 

 of the teeth are as follow : 



Lines. 



Space occupied by the three molars, . . . . .29 



Breadth of fourth molar, . . . . . . .8 



Breadth of fifth molar, . . . . • . . 8J 



Breadth of sixth molar, . . . . . . .11 



11. Several dozen isolated inferior molars, of various ages, sizes, and belonging to 

 several different species and genera, but none differing in an important degree from 

 those already indicated, from the Niobrara River. Among them is an inferior molar 

 of Equus excelsus. 



12. Two symphysial fragments of lower jaws, both having the same general form 

 and construction as in the corresponding portion of the jaw of the Horse. One is 

 much narrower in its proportions than the other, and it contains the alveoli of a con- 

 tinuous semicircle of six incisors and a pair of canines. The other, more robust in 

 its proportions and belonging to a larger animal, contains the remains of the same 



42 



