I was looking at this daguerreotype of the Delafield Commission, and where Mordecai (left) and Delafield (center) have shakos on their knees, McClellan seems to have a spiked leather helmet at his left elbow. I know American units wore these after the ACW, and I assume this one belongs to the unidentified foreign officer in the photo.
But the Crimean War seems to be a bit early for this kind of headgear, even for Europeans.
The Internet being what it is, there has to be a website devoted to the subject - and there is. Get a load of this undated photograph of the American "pickelhaube."
Meanwhile, the shape at Mac's elbow is a problem to be solved later.