I was in Arkansas when I found this unusual rock/fossil.  I thought this was a geode of some sort until I saw what appears to be bones in the center. It is exactly how I found it; I did not break it open...it was this nearly perfect, half-shell shape when I picked it up. I have a microscope that attaches to my iPhone so I have attempted to give you a good view of what I am calling "bones".  If any of you have suggestions as to what it might be, please let me know. Thank you!