Here is a specimen of a steinkern from a Cardium type shell. It is a little beat up along the sides but it could be a Cardium spillmani. Notice all the indentations that are preserved in the cast. I interpret these as molds of various worm or bryozoa fauna that colonize or parasitize old shells.
Another specimen that likely fits in the Cardium genus and could be a Granocardium tenuistriatum is shown below. It is smaller than the above specimen and in better shape.
Both specimens come from the C and D canal in Delaware but I can't say for sure what formation. They were collected from spoil piles left over from the dredging of the canal which mixed the sediment from several formations together. What I do know is that they are from the late Cretaceous (Campanian stage to Masstrichtian stage). I tried to ID the specimens from Bulletin 61, "Cretaceous Fossils of New Jersey" published by the New Jersey Geological Survey, 1958. There are two volumes of Bulletin 61: Part 1 and Part 2 .