A relatively 'primitive' feature of some sharks is the possession of bony fin spines supporting their dorsal fins.  Isolated fin spines of Heterodontus are very occasionally recorded from the Chalk, and exceptionally scare articulated remains retaining the fin spines are also known.  These moderately sized claw-like spines are smooth, curved and elongate.

1).  Heterodontus canaliculatus; portion of vertebral column with fin spines (front of individual to the right) (x0.9, Booth Museum, BMB 008555), by kind permission of John Cooper).

2).  Heterodontus canaliculatus; detail of a portion of vertebral column with fin spine (x1.9, Booth Museum, BMB 008555), by kind permission of John Cooper).

2).  Heterodontus francisci ('Horn' Shark); Dorsal fin of modern bullhead shark for illustration - distal portion of fin spine protruding vertically (front of animal to the left).  Image sourced from Getty Images.