Back to the Mahantango formation we go and here is a Goniophora. This specimen was found in the Sherman Creek member at Seven Stars.
This is a single valve of the genera that is preserved very well. Note that the rock has weathered and leached out any calcium carbonate to leave a mud stone.
This next specimen is from the same locality but is less weathered so there is still some Calcite remnant coating the shell. It's a nearly complete specimen with both valves present but missing about 1/2" of the shell. It does, however, allow and interesting diamond cross section to be seen.
One last specimen, It's a single valve and missing it's tip again but shows the characteristics that can be used to ID it. Long shell, regularly spaced and moderately raised ribs and a sharp central rib that cuts across the shell at a 45 degree angle.