Strange Preservation

[I_gotta_rock]

Here's a strange one.
I found plentiful piles of what I thought were casts and internal molds in the iron-rich St. Mary's clay of Virginia last year. Loose clam fossils riddled blocks of talus. I was able to wiggle some out of their ancient resting places without s much as scratching the matrix. Generally that's because the shell disintegrated, leaving a void between the mold and the cast. If you look closely, however, what looks here like an internal mold isn't. The sculpture on the surface is clearly the outside of the shell. If it were the inside, you would see round protrusions where the ligament attached to a depression in the shell, not concentric ridges. Shown here is the most distinct of my specimens, but not the only one. Any thoughts?
Mercenaria campechiensis, Miocene, VIrginia

[Plax]

pseudomorph. Cavity of dissolved shell filled with precipitated calcite or some such.

[I_gotta_rock]

3 hours ago, Plax said:

pseudomorph. Cavity of dissolved shell filled with precipitated calcite or some such.

If it were a pseudo morph, would there be gaps between the fossil and the surrounding matrix?

[Plax]

most of the time I'd say no. Some times you get a sort of shrunken pseudomorph of the original. Not sure of the mechanism of this.