From the album: Delaware Fossils
Small section of a heteromorphic ammonite of the genus Didymoceras. Probably D. platycostatum.
Late cretaceous
Found at Reedy Point North, Delaware City, Delaware
Mt Laurel Formation
Identified from W. J. KENNEDY AND W. A. COBBAN. UPPER CAMPANIAN (UPPER CRETACEOUS) AMMONITES FROM THE MARSHALLTOWN FORMATION-MOUNT LAUREL BOUNDARY BEDS IN DELAWARE. J. Paleont., 71(1), 1997, pp. 62-73
Thanks to abyssunder and piranha for the ID help!