This fossil is somewhat rare because it was found at a depth of 60-250 meters underground. The fossil was situated above the coal seam so it might be from the Pennsylvanian Period (late Carboniferous). It was found in Webster County Kentucky USA. The mines there are part of the Eastern Interior Basin. The formations found there are Sturgis, Carbondale, and Tradewater. The limestone layers are Carthage, Madisonville, Providence, Yeargiss?, and Lead Creek.
I would identify it as Pseudorthorceras.
This next image shows two more fossils found at the same location. A cephalopod fossil that is about 17 cm long. It is next to a large crinoid stem segment.
Learn more about the geology of this area at the United States Geological Survey:
http://pubs.usgs.gov/pp/p1151h/penn.html
