Petrodus sp.
dermal denticle #1

nodule: 1.5 cm
denticle: 1.1 cm

The measurements noted because the denticle is
slightly tilted in the picture and may give the
wrong impression of size.

General abundance:  Common

These denticles formed an overlapping skin armor
that protected the sharks. These denticles are called
Petrodus. At one time they were assumed to be from
a single species of shark, but they have been found
associated with a number of different species.
"Petrodus patelliformis" is no longer considered correct.

The above is a quote from the Kansas Fossils web site

Phosphatic nodules
Fossil Menu

Pennsylvanian Fossils of Missouri