Hi all,
I found these recently along Colorado's front range in the Niobrara Fm, about 20 mins southwest of downtown Denver. I am familiar with inoceramus, but these pieces have ridges - something I didn't think inoceramus had. They also have a similar cross-sectional structure with aragonite as the inoceramus fossils I've found, which makes me think it's some other type of clam or bivalve, maybe platyceramus?
The largest piece appears to have small attached bivalves.
Thanks for your help!