The Cham Lion Mask is the forty-first treasure that can be found in Uncharted 2: Among Thieves.
The Cham Lion Mask is found in Chapter 12 - A Train to Catch. After you jumped over to the ramp Elena lowered for you, be sure to take the time to acquire yet another treasure. This one is located on the far side of the previously-locked door, the one you had to shoot the wooden beams from off of. Instead of proceeding to the right and going along with the main quest, jump back to the left and grab the treasure from the little alcove next to the now-open door. Simple.