Paleozygopleura is a somewhat common gastropod in the Mahantango formation but it is difficult to get nice specimens as the shells must have been somewhat thin and delicate and prone to being crushed. This past winter I visited and outcrop of the Mahantango formation north of Harrisburg, Pennsylvania and got lucky when cracking rocks.
Here is the mold and cast of the shell side by side.
The detail has been nicely preserved.
At the locality I was collecting the Mahantango is somewhat sandy and is referred to as the "Montebello sandstone". Much of the rock layers exposed are indeed sandstone of varying grain sizes but there are occasional thin layers of shale that represent rises of sea level called transgressions. Another theory is the facies (rock type) change is due to the settling of sediments within the local basin which leads to deeper waters.