This is a cool fossil Mosasaur jaw section that has been nicely prepared, and has several unerupted teeth exposed that were growing inside of the jaw. Mosasaurs, like dinosaurs and sharks shed their teeth frequently as they became worn, so there were always replacement teeth growing into place inside the jaw. From https://www.fossilera.com/fossils/really-cool-8-1-mosasaur-eremiasaurus-jaw-section | Fossilera