I had to ask two friends who are faculty at The Juilliard School of Music to help identify the instrument in the photo of Thomas Benjamin Clark. It is an ophicleide, a predecessor of the tuba, for which the notorious and dreaded 'tuba' part in Mendelssohn's Overture to A Midsummer Night's Dream was written. Hear the ophicliede here.
You have to admit that being the ophicliedist for the Golden Spike Ceremony at Promontory Point in 1869 gives someone a unique place in history.
George William Proctor is seen in his photo with an ophicleide: