>I agree with Andrew that the fossil is an internal cast (The shell fills
>with sediment, which hardens inside, then the actual shell dissolves away),
>but I don't think it is a cardiid.  I think it is a member of Arcidae, close
>to the the extant species Noetia ponderosa.

Arcoids have a wider ligament area, and the muscle scars are often
raised (resulting in depressions on steinkerns); the hingeline is
also straight. Also arcoids are thickshelled and the radial ribs
generally don't appear on the inner surface.

Noetis is in arcoid family Noetiidae, which has a shorter
asymmetrical hinge than Arcidae, Parallelodontidae or Cucullaeidae.