Ever since I figured out what they were, local Pinnid fossil specimens have been a personal favorite of mine. In examining the remaining shell material in these fossils, I have seen the prismatic layer on the shell hinge in at least two specimens. The specimens below show the detail.

The layer in a specimen from May 2019

Cross section view of specimen
Hinge detail in cross section.
Closer inspection of the hinge
The Sea Pen un-freed from limestone.
Prismatic layer, dark layer to the left.
The needle like structure on the top and middle left is the prismatic layer.

The layer from Sea Pen III (Likely a species of Meekopinna)

Cephalopod found in Pennsylvanian Limestone
Cephalopod found in Pennsylvanian Limestone
Cephalopod found in Pennsylvanian Limestone
Prismatic layer has eroded from the middle outside of the hinge
Cephalopod found in Pennsylvanian Limestone
The needle structure on the right side is the prismatic layer.

Further Reading