Ever since I’ve figured out what they were, local Pinnid fossil specimens are a personal favorite to find. In examining the remaining shell material in these fossils, I have been able to see the prismatic layer represented on the shell hinge in at least two specimens. The specimens below show the detail.
The layer in a specimen from May 30th, 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.like
The layer from Sea Pen III (Likely a species of Meekopinna)
Prismatic layer has eroded from the middle outside of the hinge 
The needle structure on the right side is the prismatic layer.
Further Reading
- The Prismatic Layer of Pinna: A Showcase of Methodological Problems and Preconceived Hypotheses – By Yannicke Dauphin, Alain Brunelle, Kadda Medjoubi, Andrea Somogyi and Jean-Pierre Cuif – Minerals – Volume 8, Issue 9
- Aviculopinna & Meekopinna – Our research article on local pinnids.