Wide flat shiny triangular with hints of red 'skin'

[PerthBloke556]

Found in pleistocene limestone. Kind of triangular in shape. Has a 'skin' on it that's red that's flaked off. On the photos you can see the red line that goes around the whole shape.

 

Fremantle Australia 

 

[ClearLake]

Interesting piece. I see a number of things going on in it. Pictures 1 and 2 remind me of a pen shell (family Pinnidae) sticking out of the right end. Picture 3 has what looks like a ribbed bivalve in the lower center with perhaps a couple of other mollusks in there too. Not sure what picture 4 is showing.