Badlands Fossil ID

[MacHoffman22]

I found this fossil today in the badlands of south Dakota in a crevice at the bottom of a butte. I did not remove it but am impatient and with the photos I supplied, the paleontologist at the visitors center had a hard time identifying it and I am extremely curious to know what it may be. The round part of the bottom picture continues to be almost perfect oval through the top, however a chunk had broken off there. The discolored brown line in the crevice in  the center of the first picture appeared to be bone that just barely breached the surface of the specimen (too hard to picture).