The skin of sharks is armoured with small dermal denticles formed from dentine, known as placoid scales.  These are of varying size and density,  forming a scaly cover in some taxa, and a thorny defensive ornament in others.  These have good preservation potential, but are relatively uncommon in Chalk collections, perhaps due to their typically small to microscopic size.   

A

B

1).  Squatina cranei; (A,B) Details of the type specimen showing dermal denticles and other elements embedded in the skin of the animal in life (x6.5, Grey Chalk, Clayton, near Brighton, Sussex, Willett Collection, Booth Museum, BMB 007330, by kind permission of John Cooper).

2). Squatina cranei; Detail of the type specimen showing dermal denticles resting against a calcified formerly cartilaginous skeletal element from the back of the head (x9).

3).  Thorny placoid scales on the tail of a modern Rajid (skate) (x4).  Image courtesy of Charlie Underwood.