Your ammonite probably is...

Oxybeloceras

Oxybeloceras is very similar to Ptychoceras and Solenoceras (less so to Hamulina and Hamites). Like these genera, it is normally found in fragments. Oxbeloceras had a tightly coiled protoconch, but then grew straight out, only to turn 180 degrees and grow straight in the opposite direction. Ptychoceras does not have this coiled protoconch, while Solenoceras has a collar around its aperture, which Oxybeloceras lacked. Oxybeloceras is from the Upper Cretaceous of North America.