Maastrichtian Pycnodonte sp. ?

[Hipockets]

Hello,    I have recently found these pycnodontes at a maastrichtian site in SE North Carolina, USA . It is Pee Dee formation. Would anyone know the speices ? The upper valve is rather flat as opposed to concave and has radiant grooves which I have not seen in my research. The lower valves vary from being almost flat on some to a deep dish concave on others. I have attached two different ones. The first picture of each is the upper valve showing the radiant grroves. Second picture is lower valve. Third picture is the hinge area. Fourth picture is an attempt to show the overall contour. Thanks for any help. 

[Plax]

I'm used to Pynodonte vesicularis or mutabilis being rather "deep dish". Looking at the series you showed me there are some a little deeper in the lower valve but not comparable to others I've seen locally in the Peedee. the odd shallow depth of the lower valve may be the result of the surface attachment. A big flat pelecypod shell, like inoceramus for instance, may make for a flat lower valve. When you google this oyster and look at images there are none that I saw with the radial lines on the upper valve. Richards in Cretaceous of New Jersey volume 1 mentions them though in the text. Oyster experts are few in my limited experience but Charles Newsome comes to mind if he's still active.

  

[Plax]

Just had another thought. Slow sedimentation rates may also make for a flatter lower valve if the depth of the valve is a result of potential burial.

[Hipockets]

Update...........Dr. Charles Newsom has identified them as Pycnodonte convexa. He said they are really varible.