Hi James,
Thank you for your inqury. Based on some research the productida Echinoconchus sp are younger and found in a different region. The examples of Echinoconchus sp lack or do not show signs of spines.
Where as my specimens have spines projecting off the pedicle valve and pedicle opening. These spines are long. I have seen these spines extend a few inches into the matrix. The pedicle valves are very wrinkled. These productida are found in western edge of the late Mississippian age silty shale which are part of the Big Snowy Group. These rocks were once part of the edge of a large sea or tidal lagoon.
If you right click the posted image to view photo, you can them view the image in magnification to see spines and valve details. The photo shows both valves of this type of productida.