Dear Igor,
This could be a part of a bivalve shell. In the family Pholidae there is a "apophysis" inside the hinge part. Take a look at species like Zirphaea crispata or Pholas spp. In your Pictures it looks as if the structures shown are detacheable. I think that the apophysis in Pholidae are fixed which would contradict my idea about an pholid apophysis. Anyway check with your marine biologists and take a look at modern specimens.
Leif Jonsson / GNM, Gothenburg, Sweden
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