E. L. TroxelL — Early Pliocene One- Toed Horse. 345 



Comparative measurements of types : P. lullianus nov. P. pemix Marsh 



ratio % 



Ant-post, diameter of wolf tooth .. 16 81 18 



Ant-post. diam. of first upper true molar 27 81 22 

 Ant-post. diam. of second deciduous molar 36 



86 



" " " " premolar 31 



Extent of four upper deciduous molars . 107 



82 



« " « " premolars 88 



Extent of three lower deciduous molars. 89 



83 



" " " " premolars 74 



Length of radius 263 96 253 



Width of proximal end 59 88 49 



Width of articulation, distal ... 44 81 40 



Length of cannon bone 219 86 189 



Length of first phalanx 56 98 55 



Length of second phalanx 26 96 25 



Length of hoof bone 36 106 38 



Width of hoof... 40 125 50 



Length of skull 380 108 410 



III. Geology of the Oak Creek Formation. 



The skeleton of the new type was found in the eastern part 

 of the Rosebud Indian Reservation, near the town of Mission, 

 South Dakota. This Reservation, at least the western part in 

 the Miocene formation of the valley of the Little White River, 

 has long been a favorite hunting ground for specimens of 

 extinct animals. 



The region east of Mission is slightly rolling but a very fer- 

 tile farming country. Because it is so productive of vegetation, 

 it was generally considered an unprofitable place to hunt for 

 fossils, for only at infrequent intervals along the crests of the 

 hills is the bed rock of the later formations well exposed. 

 Most of this land has been allotted to the Indians, whose pres- 

 ent peaceful life is in great contrast to that of the time of 

 Marsh and the other early explorers. 



A table land, whose northern escarpment extends in a direct 

 line for quite a distance and rises about 200 feet above the 

 valley floor, forms a divide between the Iveyapaha and the 

 valley of Oak Creek. In nearly every direction the fiat topped 

 hills can be seen, all apparently conforming in height. 



Few geologists have visited this area, but one, A. B. Reagan,* 

 while carrying on his missionary work among the Indians, paid 

 much attention to the earth formations and collected many 



* Albert B. Eeagan, American Geologist, vol. xxxvi, pp. 229, 1905. 



