I had initially posted a pic of this in Fossil ID and claimed that I had lost it in the field. Well, turns out that I didn't lose this neat Hederella after all! I found it at the bottom of one of my trip buckets.
Yeah, I know... I always find the best and coolest Platyceras. This one is a P. arkonense. The other one at this site, P. conicum, looks like, well, a wrinkled cone.