Agomphus tardus was a turtle that probably lived in a lake or river and washed out to sea after it died.
Agomphus tardus, 7 costals (side), 1 peripheral (edge) and a significant portion of the plastron (bottom). This specimen was donated to the New Jersey State Museum. Image and info courtesy of John Whitley, http://www.fossilsofnj.com/trips/inversand_turtle.htm