Yes, Charlie's right. It's a section of fossil tooth plate from a member of the ray family.
Compare the detail on the first picture below with the arrangement in the specimen circled in the second picture. Both are Myliobatis species (Eagle Rays) and the exact arrangement depends on both the species and the position in the mouth.
Likely from Miocene deposits along the Potomac. It is much more common to find the fragments of the plate than to find the plate more intact - as you have.
Congrats on a nice find. You will find many fossil shark teeth mixed in those deposits with the ray plates, as well as fossil bivalves and gastropods.