George Gershwin scored "An American in Paris" for full symphony orchestra plus some unconventional instruments. Which one of these four was not written into the score?

Gershwin composed An American in Paris on commission from the conductor Walter Damrosch. He scored the piece for the standard instruments of the symphony orchestra plus celesta, saxophones, and automobile horns. He brought back some Parisian taxi horns for the New York premiere of the composition, which took place on December 13, 1928 in Carnegie Hall, with Damrosch conducting the New York Philharmonic. Gershwin completed the orchestration on November 18, less than four weeks before the work's premiere. If you don't know it, why not listen to this recording of the NY Philharmonic conducted by Leonard Bernstein?