Balls, apparently. You often see ammonites cracked or eroded open showing the mineralized body compartments (the phragmacone chambers) but I have never before seen these. The Hilderocas on the right of this pair has hard mineral balls in the middle of the fossil. The fossil is heavily flattened and coudn't be recovered in one piece: the balls do extend through the fossil.
Anyone any ideas? Hitherto unseen innards? An artifact of the fossilization process? The results of a prehistoric parasitic infection?