Bony symmetrical plate. Please help with correct I.D.

[hahnewald]

This was washed up by the sea, where shark teeth, pieces of bone, sperm whale teeth, whale inner ear bones and several other common vertebrate fossils can be seen. Location is South Africa, West Coast. Does anybody know the correct I.D. for this particular piece? I thought I had I.D.'d it but i'm not convinced any more. Thanking you in advance.