This is a gastropod fossil called Platyceras bistrami from the Belen formation (Devonian, Emsian stage) of Bolivia. It seems to be distinguished from other species by the undulating surface of it's shell. This is just a cast of the shell but the ripples are preserved well. Also, P. bistrami (when I write it that way it makes me think of Pastrami) has a tighter coil to it's shell than other species.
This specimen was found south of La Paz, Bolivia and I purchased it off of E-bay.