Petrodus sp.
dermal denticle #3
weathering out of a phosphatic nodule

  

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